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Elizabeth  Stone 


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THIS  BOOK  MUST  NOT  BE  TAKEN 
FROM  THE  LIBRARY  BUILDING. 


25M/1 -88-880256 


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T  H  K  / 


ANATOMY  OF  THE  HORSE 


DISSECTION     GUIDE 


J.  M' FAD  YEAN,  M.B.,  CM.,  B.Sc, 

MEMBER   OF   THE    ROYAL   COLLEOK   OF   VETERINARY    SURCiEONS, 
LECTl'RER    ON    ANATOMY    AT   THE   ROYAI.   (diCK's)   VETERINARY    COLLEfiE,    EDINBURGH. 


WILLIAM    R    JENKINS, 

VETEKINARY    PUBLISHER    AND    BOOKSELLER, 
850   SIXTH   AVENUE,    NEW   YORK. 


AND   A.    K.    .lOHNSTON,    I'RINTKKS,    KDISBl'KliU    AND    LONDON. 


TO 

WILLIAM   TURNER,   M.B.,    LL.D,    F.R.S. 

PROFESSOR    OF    ANATOMY    IN    THE    UNIVERSITY    OF    EDINBURGH, 
AS    A    TRIBUTE   TO    HIS    EMINENCE 

AS    AN    ANATOMICAL    TEACHER. 


PEEFAOE. 


The  want  of  an  illustrated  topographical  treatise  on  equine  anatomy  has, 
in  the  experience  of  the  author,  been  a  great  barrier  to  the  efficient 
teaching  of  that  all-important  branch  of  veterinary  education.  In  this 
work  the  object  of  the  author  has  been  to  place  in  the  hands  of  veterinary 
students  a  dissection  guide  comparable,  in  some  degree,  to  the  text-books 
at  the  service  of  the  practical  student  of  human  anatomy.  The  order  of 
dissection  laid  down  is  that  which  the  author  has  found  to  be  most  advan- 
tageous, and  he  has  attempted  to  describe  with  accuracy  and  moderate 
fulness  the  different  organs  as  they  present  themselves  in  that  order. 
This  description  is  largely  supplemented  by  the  illustrations,  which  arc 
so  complete  that  almost  every  organ  in  the  body  is  delineated.  The 
majority  of  these  illustations  are  original,  being  faithftil  portraits  of  the 
author's  own  dissections.  It  is  hoped  that  they  will  prove  useful  to  the 
student,  in  the  first  place,  as  a  plan  and  a  guide  in  his  work,  and, 
secondly,  as  a  means  by  which  he  may  afterwards  summon  up  a  mental 
picture  of  his  own  dissections. 

While  the  book  is  specially  designed  for  use  in  the  dissecting-room, 
the  author  ventures  to  hope  that  it  may  also  be  serviceable  to  the 
veterinary  practitioner.  Special  care  has  been  taken  in  portraying  those 
regions  that  possess  a  surgical  interest,  and  the  illustrations  furnish  a 
ready  means  by  which  the  surgeon  may  refresh  his  memory  regarding 
the  objects  to  be  met  in  the  course  of  an  operation. 

The  greater  number  of  the  original  drawings  were  made  in  the  dissecting- 
room  of  the  Royal  (Dick's)  Veterinary  College,  by  Mr  J.  Bayne,  artist; 
a  few  were  executed  by  Mr  R.  S.  Reid,  artist;  and  the  remainder  by 
Mr  R.  H.  Potts,  veterinary  student.  A  few  of  the  illustrations  were 
directly  drawn  on  stone  by  Messrs  W.  &  A.  K,  Johnston.  To  all  of 
these  gentlemen  the  author  is  miich  indebted  for  the  clearness  and 
fidelity  with  which  they  have  delineated  the  various  objects. 

The  source  of  each  of  the  borrowed  illustrations  is  duly  acknowledged 
elsewliere,  but  the  author  is  constrained  to  make  special  mention  of 
those  from  the  systematic  text-book  of  Professor  Chauveau,  who 
generously  consented  to  the  copying  of  as  many  (tf  his  figures  as  might 
be  thought  usefid  for  this  work. 


To  insure  acciinicy,  the  author  has  been  careful  to  compare  tlie  results 
of  his  own  dissections  with  the  descriptions  of  other  writers,  and  more 
especially  with  the  works  of  Percivall,  Le^'h,  and  Chauveau,  to  which  he 
begs  to  express  his  indebtedness. 

To  Professor  Turner  the  author  is  luider  deep  obligation  for  the 
revision  of  the  chapters  on  the  brain,  the  eye,  and  the  ear,  and  for  much- 
esteemed  suggestions  regarding  other  points. 

Finally,  the  author's  best  thanks  ai-e  due  to  his  brothers  (lavin  and 
Andrew,  who  have  carefully  revised  the  entire  proof-sheets,  and  to 
Mr.  T.  Barker,  veterinary  student,  for  assistance  in  making  the  index. 


ROYAI.  (DiCK'S)    VeTKRINAKY   COLLEGK,    EniNBUUGH, 

Octoljer,  1884. 


A  few  words  are  here  necessary  in  explanation  of  the  system  of  nomen- 
clature used  throughout  this  work.  Although  reluctant  to  add  to  the 
confusion  already  prevailing  in  the  nomenclature  of  veterinary  anatomy, 
the  author  has  not  confomied  to  any  of  the  systems  in  general  use. 
The  system  here  employed  is  based  on  the  principle  of  naming  each 
object  after  the  homologous  object  in  human  anatomy.  So  far,  indeed, 
as  any  of  the  systems  in  use  can  be  said  to  follow  a  principle,  it  is  that 
just  stated ;  but  the  violations  of  the  principle  are  numerous,  and,  in 
most  cases,  appear  to  have  been  dictated  by  the  merest  caprice.  The 
most  vicious  form  of  depai-ture  from  the  principle  is  that  in  which  terms 
adopted  from  human  anatomy  are  employed  to  designate  not  the  actual 
homologues,  but  other  parts  having,  it  may  be,  some  fiiint  resemblance 
in  shape  or  otherwise  to  the  objects  bearing  these  names  in  the  human 
subject.  This  method  is  indefensible,  since  it  tends  to  produce  the 
greatest  confusion,  and,  if  generally  adopted,  would  render  a  comparison 
of  the  anatomy  of  any  two  animals  an  impossibility.  Many  such  terms 
have  long  been  in  use,  but  it  is  hoped  that  they  are  not  ineradicable. 

In  cases  where  objects  appear  to  be  without  homologues  in  human 
anatomy,  new  names  must,  of  course,  be  found.  In  only  a  few  of  these 
instances,  however,  has  the  author  employed  terms  of  his  own  invention, 
preferring,  in  general,  to  adopt  some  of  those  already  in  use. 

The  greatest  diversity  of  names,  it  will  be  found,  exists  in  the  case  of 
muscles,  and  the  following  table  of  synonyms  has  been  compiled  for  the 
convenience  of  those  already  familiar  with  the  terms  employed  in  some 
other  works. 


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PLATES. 


Plate. 

1.  Pectoral  Region 

2.  Pectoral  Region 

3.  Brachial  Plexus 

4.  Shoulder — Outer  Aspect     . 

5.  Shoulder  and  Arm — Inner  Aspect 
G.   Shoidder,  Arm,  and  Fore-arm — Inner  Aspect 

7.  Fore  limb — Outer  Aspect    . 

8.  Shoulder,  Arm,  and  Fore-arm — Outer  Aspect 

9.  Metacarpus  and  Digit— Inner  Aspect 

10.  The  Foot       .... 

11.  Joints  and  Ligaments  of  Fore  limb 

12.  Thigh — Inner  Aspect 

13.  Thigh — Inner  Aspect 

14.  Thigh — Inner  Aspect 

15.  Hip  and  Thigh 

16.  Hip  and  Thigh 

17.  Leg — Inner  Aspect 

18.  Leg — Outer  Aspect 

19.  Metatarsus  and  Digit — Outer  Aspect 

20.  Chest-wall  and  Back 

21.  Chest-wall  and  Back 

22.  Thoracic  Cavity— Left  Side 

23.  Heart  and  Great  Vessels— Left  Side 

24.  Heart  and  Great  Vessels — Right  Side 

25.  Thoracic  Cavity — Right  Side 

26.  Thoracic  Cavity— Right  Side 

27.  Neck  and  Intermaxillary  Space 

28.  Neck  .... 

29.  Side  of  Face 

30.  Cavity  of  the  Mouth 

31.  Cavity  of  the  Mouth,  Pharynx,  etc. 

32.  Tongue,  Pharynx,  etc. 

33.  Brain — Inferior  Aspect 

34.  Brain — Siqierior  Aspect     . 

35.  Brain — Lateral  and  4th  Ventricles,  etc. 

36.  Brain — Ganglia  of  the  Base 

37.  Male  Perinseum 

38.  Abdominal  Wall      . 

39.  Abdominal  Wall      . 

40.  Abdominal  Wall 

41.  Intestines  and  Anterior  Mesenteric  Arter; 

42.  Intestines  and  Mesenteric  Arteries 

43.  Ccelian  Axis,  etc. 

44.  Abdominal  Viscera,  etc. 

45.  Sublumbar  Region  and  Diaphragm 

46.  Male  Pelvis 

47.  Genito-urinary  Organs  of  IMale 

48.  Lumbo-sacral  Plexus 


Page. 

Fariiuj  2 

.  4 

.  6 

.  9 

.  12 

.  14 

.  18 


47 
57 
59 
61 
64 
66 
71 
74 
78 
95 
96 
105 
107 
112 
116 
119 
145 
148 
181 
185 
190 
199 
243 
245 
249 
252 
276 
287 
289 
291 
304 
306 
316 
319 
322 
342 
345 
349 


CONTENTS. 


CHAPTER  I. 


111SSECTION   OF  THE  ANTERIOR   LIMB. 


The  Pectoral  Kegion  and  tlie  Axilla 

The  Outer  Scapular  Region 

Inner  Aspect  of  the  Shoulder  and 

Arm     .... 
Outer  Aspect  of  the  Shoulder  and 

Arm     .... 
The  Fore-arm 
The  Metacarpus  and  Digit 


Page. 

Page. 

1 

The  Foot          .... 

35 

8 

The  Shoulder -joint 

43 

The  Elbow-joint      . 

44 

10 

The  Knee         .... 

46 

The  Fetlock-joint    . 

.50 

17 

The  Pastern-joint    . 

52 

19 

The  Coffin-joint       . 

52 

27 

CHAPl 

DER  II. 

DISSECTION   OF  THE  POSTERIOR  LIMB. 


The  Inner  Aspect  of  the  Thigh        .  50 
The  Hip  and  Outer  Aspect  of  the 

Thigh 03 

The  Leg 70 


The  Metatarsus  and  Digit       .         .  77 

The  Stifle-joint        ....  81 

The  Tarsus 80 

The  Tarso-metatarsal  articulation  .  90 


CHAPTER  III. 


DISSECTION   OF  THE   BACK   AND   THORAX. 


The  Cliest-wall  and  Back 
The  Cavity  of  the  Thorax 

The  Lungs 

The  Pericardium     .... 

The  Heart 

The  Nerves  and  Vessels  of  the  left 

side  of  the  Thorax 
The  Nerves  and  Vessels  of  the  right 


94 

Examination  of  the  Lung        .         .     122 

100 

Dissection  of  the  Heart           .         .     123 

103 

Structure  of  the  Heart   .         .         .     129 

105 

Articulations  of  the  Ribs         ,         .     132 

106 

Intervertebral    Joints    and    Liga- 

ments            134 

108 

The  Spinal  Cord      ....     130 

side  of  the  Thorax 


.     116 


CHAPTER  IV. 


DISSECTION  OF  THE   HEAD   AND   NECK. 


The  Under  part  of  the  Neck  . 
The  Ujiper  part  of  the  Neck  . 
Ligaments  and  Ai-ticidations  of  the 

Neck  posterior  to  the  Dentata 
The  External  Ear   . 
The  Parotideal  Region    . 
The  Region  of  the  Poll   . 
The  Intermaxillary  Space 
The  Appendages  of  the  Eye    . 
The  Face  .... 

The  Pterygo-maxillary  Region  and 

the  Region  of  the  Guttural  Pouch 


Page. 
143 
151 

158 
159 
164 
168 
170 
172 
176 

184 


Page. 

The  Tongue 195 

The  Hard  Palate     .         .         .         .200 
The  Soft  Palate       .         .         .         .202 
The  Pharynx,  the  Hyoid  Bone,  and 
the  Base  of  the  Skull  ...     204 

The  Orbit 208 

The    Occipito  -  atlantal    Articula- 
tion       214 

The  Atlanto-axial  Articulation  .  215 
The  Temporo-maxillary  Articulation  215 
The  Cavity  of  the  Nose  .        .        .216 


CHAPTER  V, 


DISSECTION   OF  THE   LARYNX, 


Cartilages  of  the  Larynx 
Articulations,  Ligaments,  and  Mem- 
branes of  the  Larynx  . 


Membranes  of  the  Brain 
Arteries  of  the  Brain 
The  Medulla  Oblongata . 
The  Pons  Varolii     . 


224 

226 

Muscles  of  the  Larynx    . 
Nerves  of  the  Larynx      . 
Interior  of  the  Larynx     . 

.  228 
.  230 
.     230 

CHAPTER  VI. 

CTION  OF  THE  BRAIN. 

2.33 

236 
238 
240 

The  Cerebellum       . 
The  Cerebrum 
The  Cranial  Nerves 

.  240 
•  242 
.     253 

CHAPTER  VII. 

DISSECTION  OF  THE  EYEBALL. 


Dissection  of  the  Eyeball 257 


CHAPTER  VIII. 


The  Middle  Ear 


DISSECTION  OF  THE  EAR. 

.    267       I      The  Internal  Ear 


CHAPTER  IX. 


DISSECTION  OF  THE  PERIN/EUM  IN  THE  MALE. 


ThePerinsBum         .        .        .        .     274       I      The  Prepuce 

The  Scrotum 277       |      The  Penis 

The  Testicle  and  Epididymis  ,        .     279 


281 


CONTENTS. 
CHAPTER  X. 


DISSECTION  OF  THE   ABDOMEN. 
Page. 


The  Abdominal  Wall      .  .  .285 

The  Cavity  of  the  Abdomen  .  .     294 

The  Sublumbar  Region  .  .  .320 

Structure  of  the  Stomach  .  .     328 


Structure  of  the  Liver  , 
Structure  of  the  Spleen  . 
Structure  of  the  Pancreas 
Structiure  of  the  Kidney 


Page. 
,  330 
,  332 
.  333 
.     333 


CHAPTER  XI. 


DISSECTION  OF  THE  PELVl 


The  Hip-joint  and  the  Ligaments 
of  the  Pelvis         .... 
The  Cavity  of  the  Pelvis 
Reproductive  Organs  in  the  Female. 


340 
351 


The  Tail 360 

Joints     and     Ligaments    of     the 
Sacrum  and  Coccyx     .        .        .     363 


Fig. 
1. 

2. 
3. 
4. 

5. 


Pa( 


Dissection  of  the  Metacarpus  and  Digit,   showing  the  Tendons  and  their 
Synovial  Sheaths  .  .  .  .  .  .  .  .33 

Muscles  of  the  Tail,  deep  Muscles  of  the  Hip,  and  Pelvic  Ligaments     .  .     68 

Flexor  Metatarsi  Muscle     .  .  .  .  .  .  .  .76 

Femoro-tibial  Ligaments     .  .  .  .  .  .  .  .83 

Ligaments  of  the  Tarsus      .  .  .  .  .  .  .  .87 

Diagrams  showing  Disposition  of  the  Pleurae        .....  101 

Floor  of  the  Thorax .120 

Termination  of  the  Air  Passages  in  the  Lung        .....  122 

Diagram  of  the  two  Cavities  of  the  Right  Side  of  the  Heart       .  .  .  125 

Diagram  of  the  two  Cavities  of  the  Left  Side  of  the  Heart         .  .  .  128 

Root  of  the  Common  Aorta  laid  open        ......  129 

View  of  a  partial  Dissection  of  the  Fibres  of  the  Left  Wall  of  the  Ventricles  in 

a  Sheep's  Heart 131 

Two  cos  to-vertebral,  and  two  intervertebral  Joints,  viewed  from  below  .  132 

Two  costo-vertebral,  and  two  intervertebral  Joints,  viewed  from  above  .  133 

View  of  the  Membranes  of  the  Spinal  Cord  .....  138 

Portion  of  Spinal  Cord  with  the  Roots  of  the  Nerves       ....  139 

Transverse  Section  of  Spinal  Cord  of  Calf  .....  141 

Ligamentum  Nuchaa  and  Deep  Muscles  of  the  Neck         ....  156 

Auricular  Muscles  and  Nerves  of  a  Mule  ......  161 

Cartilages  of  the  Nose         ........  176 

Hard  Palate 201 

Muscles  of  the  Eyeball 209 

Muscles  of  the  Eyeball         .  .  .  .  .  .  .  .210 

Longitudinal  Section  of  the  Head,  showing  the  Cavities  of  the  Mouth,  Nose 
and  Pharynx         .  .  .  .  .  .  .  .  .217 

Transverse  Section  through  the  Nasal  Chambers  ....  218 

Larynx,  side  view    .........  228 

Larynx,  side  view    .........  229 

Larynx,  back  view  .........  230 

Interior  of  the  Larynx,  seen  from  behind  .....  231 

Arteries  of  the  Brain  ........  236 


XX  CONTENTS. 

Paoe. 

31.  Corpus  CallosuiT)  and  inner  face  of  cerebral  Hemisphere.  .  .  .  248 

32.  View  of  the  Lower  Half  of  Kight  Adult  Human  Eye,  divided  horizontally 

through  the  middle  .  .  •  .  .  .  .  .  .258 

33.  Choroid  Membrane  and  Iris  exposed  by  the  removal  of  tlie  Sclerotic  and 

Cornea       .  .  .  .  .  .  •  •  •  .261 

34.  Diagram  of  the  membranous  Laljyrinth    ......  269 

35.  Transverse  Section  through  the  Tube  of  the  Cochlea       .  .  .  .271 

36.  ^''ertical  Section  through  the  Wall  of  the  Duodenum,  showing  the  Glands  of 

Brunner    ..........  309 

37.  Diagrammatic  "View  of  a  small  portion  of  the  Mucous  Membrane  of  the  Colon    310 

38.  Vertical  Section  of  the  Coats  of  a  Pig's  Stomach  ....  329 

39.  Stomach,  Everted  and  Inflated       .  .  .  .  .  .  .330 

40.  Longitudinal  Section  of  a  Portal  Canal,  from  the  Pig      ....  331 

41.  Transverse  Section  through  the  Hepatic  Lobules  ....  332 

42.  Cut  Surface  of  Horse's  Spleen,  Trabecular  Framework    ....  333 

43.  Vessels  of  tlie  Kidneys  and  Uriniferous  Tubules  ....  335 

44.  Bladder  and  Intrapelvic  portion  of  the  Urethra  opened  from  l^elow        .  .  348 

45.  Section  of  tlie  Cat's  Ovary  .  .  .  .  .  .  .353 

46.  Right  Ovary  and  Fallopian  Tube    .  .  .  .  .  .  .354 

47.  Generative  Organs  of  the  INIare,  viewed  from  above         ....  356 

48.  Muscles  of  the  Tail,  Deep  Muscles  of  the  Hip,  and  Pelvic  Ligaments    .  .  361 


CHAPTER    I. 

DISSECTION   OF  THE  ANTERIOR  LIMB. 

THE    PECTORAL    REGION    AND    THE    AXILLA. 

As  the  first  step  in  the  examination  of  the  fore  limb,  the  student  should 
dissect  the  structures  Avhich  pass  between  the  trunk  and  the  ventral 
aspect  of  the  limb. 

Position. — The  subject  should  be  placed  on  the  middle  line  of  its 
back,  and  its  limbs  should  be  forcibly  drawn  upwards  and  outwards  by- 
ropes  running  over  pulleys  fixed  to  the  ceiling.  If  only  one  side  is 
being  dissected,  the  subject  may  be  inclined  as  in  Plate  1.  This  will  put 
the  muscles  and  other  sti'uctures  on  the  stretch,  and  thus  facilitate  their 
dissection. 

Stc7 face-marking. — In  the  fore  part  of  the  pectoral  region  the  student 
will  notice  the  well-marked  prominence  formed  by  the  anterior  super- 
ficial pectoral  muscle.  Between  this  muscle  and  the  lower  edge  of  the 
mastoido-humeralis  there  is  a  groove  in  which  will  afterwards  be 
dissected  the  cephalic  vein  and  a  branch  of  the  inferior  cervical  artery. 
Extending  inwards  from  the  point  of  the  elbow  is  a  prominent  fold  of 
skin  over  the  hinder  edge  of  the  posterior  superficial  pectoral  muscle. 

Dh'ections. — An  incision  through  the  skin,  but  not  deeper,  is  to  be 
made  along  the  middle  line  of  the  sternum,  from  the  ensiform  cartilage 
as  far  forwards  as  the  cariniform  cartilage.  From  the  middle  of  this 
incision  another  is  to  be  carried  transversely  outwards,  and  terminated 
a  little  beyond  the  elbow-joint.  Where  this  second  incision  stops, 
another  is  to  be  made  across  the  inner  face  of  the  fore-arm.  Beginning 
at  the  point  where  these  incisions  meet,  the  student  should  raise  and 
turn  outwards  the  two  flaps  of  skin,  so  as  to  denude  the  superficial 
pectoral  mviscle.  In  doing  this,  it  may  be  noticed  that  here,  as  in  other 
unexposed  situations,  the  skin  is  comparatively  thin.  Beneath  the  skin  is 
the  subcutaneous  fascia,  and  search  is  to  be  made  in  it  for  the  cutaneous 
nerves  of  this  region. 

Cutaneous  Nerves,  A  nerve  of  considerable  size,  derived  from  the 
6th  cervical  nerve  (Plate  1),  crosses  the  groove  between  the  mastoido- 
humeralis  and  the  anterior  superficial  pectoral,  and  distributes  branches 
to  the  skin  over  the  latter  muscle  and  part  of  the  posterior  superficial 
pectoral.     Other  small  cutaneous  twigs,  which  are  branches  of  the  inter- 


::  THE  ANATOMY  OF  THE  HORSE. 

costal  nerves,  appear  near  the  middle  line,  and  are  dix-ected  transversely 
outwards. 

Directions. — The  surface  of  the  superficial  pectoral  muscles  should 
now  be  carefully  cleaned  by  the  removal  of  the  subcutaneous  fascia ; 
and  this  ojDeration  should  be  conducted  by  beginning  at  the  anterior  or 
posterior  border  of  the  muscle  and  working  parallel  to  the  direction  of 
the  muscular  fibi-es.  When  this  has  been  effected,  a  line  will  be  seen 
on  the  surface  of  the  muscle ;  and  by  dissecting  carefully  down  on  this 
line,  the  student  will  be  able  to  separate  tlie  anterior  from  the  posterior 
part  of  the  muscle.  Search  is  to  be  made,  in  the  groove  already  men- 
tioned, for  the  cephalic  vein,  and  the  f;\t  is  to  be  carefully  i-emoved  fi'om 
the  vein  and  its  accompanying  arterial  branch. 

Superficial  Pectoral  Muscle  (Pectoralis  transversxis  of  Percivall). — This 
muscle  is  divided,  though  not  very  distinctly,  into  two  portions,  which 
may  be  distinguished  as  the  anterior  superficial  pectoral  and  the  pos- 
terior superficial  pectoral. 

The  Anterior  Superficial  Pectoral  (Plate  1)  arises  from  the  first 
two  or  three  inches  of  the  inferior  border  of  the  sternum,  its  posterior 
fibres  overlapping  the  anterior  part  of  the  next  muscle.  It  is  inserted 
into  the  external  lip  of  the  musculo-spiral  gi'oove. 

The  Posterior  Superficial  Pectoral  (Plate  1)  arises  from  the  inferior 
border  of  the  sternum  from  within  an  inch  of  its  anterior  end  as  far 
back  as  a  point  behind  the  6th  costal  cartilage,  and  from  a  fibrous  cord 
which  joins  the  muscle  along  the  middle  line  to  its  fellow  of  the  opposite 
side.  It  is  inserted  into  the  superficial  fascia  which  descends  on  the 
inner  face  of  the  fore-arm ;  and  a  few  of  its  anterior  fibres,  forming 
a  band  about  one  inch  in  breadth,  are  inserted  along  with  the  preceding 
muscle  into  the  external  lip  of  the  musculo-spiral  groove.  At  the 
elbow-joint  the  muscle  covers  the  posterior  radial  vessels  and  the  median 
nerve,  but  these  are  not  to  be  exposed  at  present. 

Action. — The  superficial  pectoral  muscle  is  an  adductor  of  the  limb  at 
the  shoulder,  and  the  posterior  division  of  the  muscle  is  also  a  tensor  of 
the  fascia  of  the  fore-arm. 

Directions. — Both  divisions  of  the  muscle  are  now  to  be  cut  across  near 
their  origin,  and  dissected  carefully  from  the  subjacent  deep  pectoral ; 
and  while  this  is  being  done,  search  is  to  be  made  for  their  nerves,  which 
come  from  the  brachial  plexus  by  passing  between  the  two  divisions  of 
the  deep  pectoral  muscle.  In  x*eflecting  the  muscle,  the  dissector  will 
cut  many  small  branches  of  the  external  or  internal  thoracic  vessels. 
The  reflected  muscles  are  now  to  be  fastened  outwards  with  chain  and 
hooks,  and  the  dissection  of  the  deep  pectoral  is  to  be  undertaken  after 
the  cephalic  vein  has  been  examined. 

The  Cephalic  Vein  (Plate  1).  This  is  the  upward  continuation 
of  one  of  the  divisions  of  the  internal  subcutaneous  vein  of  the  fore-arm. 


DISSECTION    OF    THE    ANTERIOR    LIMB.  6 

It  ascends  in  the  groove  between  the  anterior  superficial  pectoral  and 
the  mastoido-humeralis.  In  the  inner  third  of  this  groove  it  lies  on  the 
anterior  deep  pectoral,  in  company  with  a  branch  of  the  inferior  cervical 
artery.  It  empties  itself  into  the  jugular  about  two  inches  from  the 
lower  end  of  that  vessel. 

Deep  Pectoral  Muscle. — This  consists  of  two  distinct  divisions,  which 
may  be  distinguished  as  the  anterior  deep  pectoral  and  the  posterior 
deep  pectoral. 

The  Posterior  Deep  Pectoral  (Pectoralis  magnus  of  Percivall)  (Plate 
2)  is  a  muscle  of  large  size ;  and  its  posterior  part,  being  subcutaneous, 
was  visible  before  reflection  of  the  superficial  pectoral.  It  arises  from  the 
abdominal  tunic  covering  the  external  oblique  and  the  straight  muscles 
of  the  abdomen  ;  from  the  tips  of  the  cartilages  of  the  5th,  6th,  7th,  and 
8th  ribs,  and  from  the  immediately  subjacent  lateral  siirface  of  the 
sternum.  It  is  inserted  into  the  inner  tuberosity  of  the  humerus,  into 
the  tendon  of  origin  of  the  biceps,  and  into  the  fascia  which  retains  that 
muscle  in  the  bicipital  groove.  By  its  deep  face  the  muscle  serves  to 
bound  the  axillary  space  ;  while  its  upper  border  is  closely  united  to  the 
panniculus,  and  bordered  by  the  subcutaneous  thoracic  nerve  and  vessels. 

The  Anterior  Deep  Pectoral  {Pectoralis  parvus  of  Percivall)  (Plate  2) 
arises  from  the  cartilages  of  the  fii'st  four  ribs,  and  from  the  immediately 
subjacent  lateral  surface  of  the  sternum ;  and,  being  carried  upwards  in 
front  of  the  supraspinatus  nearly  as  far  as  the  cei'vical  angle  of  the 
scapula,  it  is  somewhat  loosely  inserted  into  the  fascia  covering  the  last- 
named  muscle.  This  insertion  is  concealed  by  the  mastoido-humeralis, 
and  will  be  better  seen  in  the  dissection  of  the  muscles  on  the  outer 
surface  of  the  scapula  (Plate  4).  The  deep  face  of  the  muscle  forms 
part  of  the  inferior  boundary  of  the  axilla. 

Action. — The  two  divisions  of  the  deep  pectoral  have  the  same  action, 
which  is  to  pull  the  shoulder-joint,  and  thus  the  whole  limb,  backwards. 
When  the  limbs  are  fixed,  the  muscle  may  to  some  extent  act  as 
a  muscle  of  inspiration. 

Directions. — The  deep  pectoral  muscles  are  now  to  be  severed  care- 
fully about  midway  between  their  origin  and  insertion,  and  the  cut 
portions  are  to  be  turned  outwards  and  inwards.  Their  nerves, 
which  come  from  the  brachial  plexus,  will  be  found  entering  their 
deep  face ;  and  care  is  to  be  taken  of  the  external  thoracic  and 
inferior  cervical  arteries.  By  the  reflection  of  these  muscles,  the  axilla 
is  exposed.  Owing  to  the  limited  power  of  abduction  at  the  shoulder- 
j(;)int  of  the  horse,  the  dissection  of  the  space  is  attended  with  much 
greater  difiiculty  than  in  man.  The  best  method  of  procedure  is  as 
follows: — All  the  pectoral  muscles  having  been  cut  across,  the  limb  is 
to  be  forcibly  separated  from  the  chest-wall;  and,  to  permit  this  to 
a  sufficient  extent,  it  may  be  necessary  to  cut  the  mastoido-humeralis  in 


4  THE   ANATOMY    OP   THE   HORSE. 

front  of  the  shoulder.  On  looking  into  the  space,  the  dissector  Avill  now 
see  it  occupied  by  a  large  amount  of  loose,  areolar  connective-tissue, 
which  envelops  its  contents,  and  fiicilitates  the  play  of  the  shoulder  on 
the  wall  of  the  thorax.  This  areolar  tissue  must  be  cleaned  away  from 
the  axillary  vessels  and  the  brachial  plexus  of  nerves,  but  most  of  the 
branches  of  these  will  be  more  conveniently  followed  after  separation  of 
the  limb  from  the  trunk. 

The  Axilla  coiTesponds  to  the  arm-pit  of  the  hiiman  subject,  and  is 
the  important  space  across  which  the  large  vessels  and  nerves  for  the 
supply  of  the  foi-e  limb  are  transmitted. 

Boundaries  of  the  space. — In  the  natural  movements  of  the  limb,  and 
before  dissection,  the  space  can  hardly  be  said  to  have  any  existence 
except  at  its  lower  part ;  but  in  the  dissected  condition  it  may  be 
observed  to  have  the  following  boundaries.  On  its  oute?-  side  are  the 
subscapularis,  teres  major,  and  (in  part)  latissimus  dorsi  muscles.  The 
{nne7'  side  of  the  space  is  formed  by  the  anterior  part  of  the  chest-wall 
covered  by  the  serratus  magnus,  lateralis  sterni,  and  intercostal  muscles. 
Inferiorly  the  space  is  enclosed  by  the  deep  pectoral  muscles,  and  there 
the  space  is  most  extensive.  Superiorly  the  outer  and  inner  boundaries 
meet  at  the  insertion  of  the  serratus  magnus  into  the  scapula.  The 
antei^or  limit  of  the  space  may  be  taken  as  foi*med  by  the  mastoido- 
humeralis  and  the  reflected  portion  of  the  anterior  deep  pectoral ;  while 
posteriorly  the  space  is  closed  by  the  panniculus  carnosus  and  skin  where 
these  are  carried  from  the  wall  of  the  thorax  over  the  outer  aspect  of 
the  shoulder. 

The  Axillary  Artery  (Plates  3  and  5)  begins  within  the  thorax.  On  the 
left  side  it  arises  as  one  of  the  terminal  branches  of  the  anterior  aorta;  while 
on  the  right  it  is  a  branch  of  the  arteria  innominata.  It  leaves  the  chest 
and  reaches  the  axilla  by  turning  round  the  anterior  border  of  the  fii-st 
rib,  below  the  inferior  insertion  of  the  scalenus.  It  crosses  the  axillary 
space,  inclining  downwards  and  backwards ;  and  at  the  anterior  border 
of  the  teres  major  tendon  it  is  directly  continued  as  the  brachial  artery. 
In  this  course  it  gives  off"  four  vessels,  viz.,  inferior  cervical,  external 
thoracic,  suprascapular,  and  subscapular ;  but  only  the  first  two  are  to 
be  followed  at  present. 

The  Inferior  Cervical  Artery  (Plates  1  and  2)  arises  from  the  front 
of  the  axillary  where  that  vessel  turns  round  the  first  rib.  After  a  course 
of  about  two  inches  it  bifurcates,  its  superior  branch  passing  between 
the  mastoido-lnuneralis  and  the  subscapulo-hyoideus,  Avhile  the  inferior 
division  passes  into  the  groove  between  the  mastoido-humeralis  and  the 
anterior  superficial  pectoral,  where  it  has  already  been  seen  in  company 
with  the  cephalic  vein. 

The  External  Thoracic  Artery  (Plate  3)  arises  about  the  same  point 
as  the  preceding,  but  from  the  opposite  side  of  the  parent  vessel ;  and 


DISSECTION    OF    THE    ANTERIOR    LIMB.  O 

passing  backwards  in  relation  to  the  axillary  surface  of  the  deep  pectorals, 
it  distributes  branches  to  these,  and  also  to  the  superficial  pectorals.  A 
slender  branch  from  it  accompanies  the  subcutaneous  thoracic  vein  to 
the  panniculus  carnosus. 

The  Axillary  Vein  is  the  upward  continuation  of  the  brachial  vein,  and 
is,  at  its  lower  part,  posterior  to  the  artery ;  but  at  the  anterior  border  of 
the  first  rib  it  is  below  the  artery,  and  it  here  joins  the  jugulars  and  the 
axillary  vein  of  the  opposite  side,  thus  forming  the  anterior  vena  cava. 

Directions. — The  axillary  vessels  may  now  be  cut  as  they  turn  round 
the  first  rib,  and  the  limb  may  be  further  abducted  to  facilitate  the 
dissection  of  the  brachial  plexus,  which,  in  its  first  step,  should  be 
undertaken  by  the  dissectors  of  the  limb  and  of  the  neck  conjointly. 

The  Brachial  Plexus  (Plate  3)  is  composed  of  the  nerves  for  the 
supply  of  the  fore  limb.  It  is  formed  by  the  inferior  primary  branches 
of  the  last  three  cervical  (6th,  7th,  and  8th)  and  first  two  dorsal  nerves. 
These,  however,  do  not  enter  into  it  in  equal  proportions.  The  6th 
cervical  sends  only  a  very  slender  branch  to  it,  while  the  7th  and 
8th,  after  detaching  a  communicating  filament  to  the  s^anpathetic, 
are  wholly  expended  in  it.  The  1st  dorsal  is,  wuth  the  exception  of  a 
similar  communicating  filament  and  a  slender  intercostal  branch,  also 
entirely  expended  in  it,  but  the  2nd  dorsal  gives  off,  besides  the 
usual  communicating  branch,  a  considerable  intercostal  nerve  before 
joining  the  plexus. 

These  roots  of  the  plexus  converge  towards  each  other,  and  come  out 
as  a  flat  fasciculus  between  the  upper  and  lower  portions  of  the  scalenus. 
In  descending  to  this  point,  the  dorsal  roots  of  the  plexus  turn  round  the 
anterior  border  of  the  first  rib,  leaving  on  it  a  smooth  impression  near 
its  upper  end.  The  several  roots  anastomose  in  an  intricate  fashion, 
contributing  to  the  formation  of  the  various  branches  of  the  plexus,  in 
proportions  that  the  student  will  not  be  able  to  trace  accurately  in  the 
course  of  an  ordinary  dissection. 

The  manner  in  which  the  several  roots  of  the  plexus  comport  themselves  is  liable  to 
slight  variation,  but  the  following  is  probably  as  common  as  any  other  :— 

I.  The  root  from  the  6th  cervical  nerve  is  a  slender  branch  detached  from  the  division 
which  that  nerve  sends  to  aid  in  the  formation  of  the  phrenic.  Passing  obliquely  back- 
wards on  the  scalenus  muscle,  it  resolves  itself  into  three  divisions — or  rather  its  fibres 
are  traceable  in  three  groups,  viz., — 1.  To  the  suprascapular  nerve ;  2.  to  the  anterior 
root  of  the  median  ;  3.  to  join  brandies  from  all  the  other  roots  of  the  plexus  in  forming 
a  broad,  flat  fasciculus  from  which  arise  the  subscapular,  circumflex,  and  musculo-spiral 
nerves. 

II.  The  root  from  the  7th  cervical  nerve  gives  a  branch  to  the  nerve  for  the  serratus 
magnus,  and  then  divides  its  fibres  in  three  directions,  viz., — 1.  To  the  above-mentioned 
fasciculus  gi\'ing  off  the  siibscapular,  circumflex,  and  musculo-spiral  nerves ;  2.  to  the 
suprascapular  nerve ;  3.  to  the  phrenic,  anterior  root  of  the  median,  and  nerve  for  the 
anterior  deep  pectoral  muscle. 

III.  The  root  from  the  8th  cervical  nerve  gives  a  branch  to  the  nerve  for  the  serratus 
magnus,  and  then  sends  its  fibres  in  three  directions,  viz., — 1.  To  the  before-mentioned 


b  THE  ANATOMY  OF  THE  HOUSE. 

flat  fasciculus  giving  off  the  subscaimlar,  etc. ;  2.  to  join  the  cord  from  which  arise  the 
posterior  root  of  the  median,  the  ulnar,  and  the  subcutaneous  thoracic  nerve ;  3.  to  thr 
anterior  root  of  the  median  and  the  nerve  for  the  anterior  deep  pectoral  muscle. 

IV.  The  roots  from  the  1st  and  2nd  dorsal  nerves  unite  to  form  a  common  cord  which 
divides  its  fibres  in  two  directions,  viz., — 1.  To  join  tlie  above-mentioned  cord  giving  otf 
the  posterior  root  of  the  median,  etc.  ;  2.  to  join  the  broad  fasciculus  from  which  arise 
the  subscajiular,  etc. 

The  following  is  a  list  of  the  branches  of  the  plexus  : — 

1.  Tlie  phrenic  or  diaphragmatic  nerve  (in  part). 

2.  The  suprascapular  nerve. 

3.  Nerves  to  the  pectoral  muscles. 

4.  The  nerve  to  the  subscapularis. 

5.  Nerves  to  the  serratus  magnus  and  levator  scapulro  (cervical 

portion  of  the  serratus),  the  latter  ouh-  in  part. 
G.  The  circumflex  nerve. 

7.  Nerves  to  the  teres  major  and  latissimus  dorsi. 

8.  The  musculo-spiral  nerve. 

9.  The  median  nerve  (two  roots). 

10.  The  ulnar  nerve. 

11.  The  subcutaneous  thoracic  nerve. 

The  Phrenic  Nerve.  This  nerve  is  formed  by  the  union  of  two,  or 
sometimes  three,  branches.  The  inconstant  branch  comes  from  the 
5th  cervical ;  the  other  ,two  come  from  the  6th  and  7th  respectively. 
The  root  from  the  6th  nerve  gives  off  a  branch  to  the  brachial  plexus, 
and  then  unites  on  the  scalenus  with  the  root  from  the  5th — when  that 
is  present.  The  single  cord  resulting  passes  obliquely  backwards  and 
downwards,  and  at  the  lower  edge  of  the  scalenus  it  joins  with  the  root 
from  the  7th  nerve.  This  last  comes  from  the  fore  part  of  the  brachial 
plexus.  The  trunk  of  the  nerve,  as  thus  formed,  passes  backwards 
between  the  axillary  ai-tei-y  and  its  inferior  cervical  branch,  and  enters 
the  thorax.     It  is  the  motor  nerve  to  the  diaphragm. 

The  Nerves  to  the  Levator  Anguli  Scapulae  and  Rhomboideus. 
In  Plate  3  two  nerves  are  seen  at  the  upper  edge  of  the  scalenus. 
They  are  not,  strictly  speaking,  branches  of  the  brachial  plexus ; 
but  come  from  the  inferior  primary  branch  of  the  6th  nerve,  and 
pierce  the  muscle  either  together  or  separately.  They  are  distributed 
to  the  levator  anguli  scapula;,  and  the  posterior  of  the  two  is  continued 
in  that  muscle  to  reach  the  rhomboideus. 

The  Nerve  to  the  Serratus  Magnus  is  formed  by  the  union  of  two 
branches,  which  pierce  the  upper  division  of  the  scalenus  before  uniting. 
These  are  branches  of  the  7th  and  8th  nerves  respectively.  By  their 
fusion  there  is  formed  a  broad  nerve,  which  passes  backwards  on  the 
surface  of  the  serratus,  distributing  its  filaments  upwards  and  downwards. 
Before  fusion,  the  branch  from  the  7th  gives  off"  a  nerve  which  is 
distributed  to  both  the  levator  and  the  serratus. 


DISSECTION    OP   THE   ANTERIOR    LIMB.  7 

The  Subcutaneous  Thoracic  Nerve  (Plates  1  and  3)  derives  its  fibres 
from  the  dorsal  roots  of  the  plexus  and  from  the  8th  cervical,  but  princi- 
pally from  the  former.  It  accompanies  the  spur  vein  to  near  the  flank, 
being  distributed  with  perforating  intercostal  branches  on  the  deep  face 
of  the  panniculus  carnosus.  A  branch  from  it  unites  with  perfoi-ating 
branches  from  the  2nd  and  3rd  intercostal  nerves,  and  turns  round  behind 
the  limb,  to  be  distributed  to  the  panniculus  over  the  shoulder  and  arm. 
The  Nerves  to  the  Pectoral  Muscles  have  already  been  referred  to. 
The  nerve  to  the  anterior  deep  pectoral  leaves  the  fore  part  of  the 
plexus,  deriving  its  fibres  from  the  7th  and  8th  cervical  nerves.  The 
nerve  to  the  superficial  pectoral  muscle  (both  divisions)  derives  its  fibres 
from  both  roots  of  the  median.  In  general,  there  are  two  nerves  to  the 
posterior  deep  pectoral.  The  first — to  the  anterior  part  of  the  muscle, 
comes  off  with  the  posterior  i*oot  of  the  median,  the  other — to  the 
posterior  part  of  the  muscle,  comes  off  in  common  with  the  subcutaneous 
thoracic. 

Directions. — The  remaining  nerves  of  the  brachial  plexus  can  be  more 
satisfactorily  followed  after  sepai-ation  of  the  limb  from  the  trunk,  and 
the  dissector  should  therefore  now  proceed  as  follows  : — Pass  a  cord 
round  the  nerves  of  the  plexus  as  they  emerge  from  between  the  two 
divisions  of  the  scalenus,  and  then  cut  the  roots  of  the  plexus  as  near 
their  points  of  origin  as  possible.  Cut  also  the  axillary  artery  and  vein 
at  the  fiz-st  rib.  This  will  allow  the  limb  to  be  carried  well  out  from 
the  trunk,  so  as  to  expose  the  serratus  magnus  and  levator  anguli 
scapulae,  which  are  now  to  be  cleaned. 

Serratus  Magmis  and  Levator  Anguli  Scajmlce. — These  muscles  are, 
in  the  horse,  not  very  distinctly  marked  off  from  each  other,  and  have 
therefore  been  frequently  described  as  one  muscle  under  the  first 
name. 

The  Serratus  Magnus  (Plate  4)  arises  from  the  outer  surfaces  of  the 
eight  (or  nine)  anterior  ribs,  its  eight  slips  of  origin  forming  a  curved, 
serrated  line  which  gives  to  the  muscle  its  name.  The  posterior  four 
of  these  slips  inter-digitate  with  slips  of  origin  of  the  external  oblique 
muscle  of  the  abdomen  (Plate  39),  and  are  overspread  by  the  abdominal 
tunic.  It  is  inserted  into  a  triangular  area  on  the  ventral  surface  of  the 
scapula  near  its  dorsal  angle,  and,  in  common  with  the  next  muscle,  into 
another  triangular  area  at  the  cervical  angle. 

Action. — It  pulls  the  dorsal  angle  of  the  scapula  downwards  and 
backwards  on  the  chest- wall,  causing  the  shoulder-joint  at  the  same 
time  to  move  upwards  and  forwards ;  but  when  the  limbs  are  fixed,  it 
can  become  a  muscle  of  inspiration,  pulling  the  ribs  upwai-ds  and  for- 
wards. In  the  standing  posture  of  the  animal  at  I'est,  the  chest  is,  in 
a  manner,  slung  on  the  fore  limbs  by  means  of  the  right  and  left  ser- 
ratus muscles. 


8  THE  ANATOMY  OF  THE  HORSE. 

The  Levator  Anguli  Scapula  (Plate  4)  arises  from  the  trans- 
verse processes  of  the  last  four  cervical  vertebra) ;  and  its  fibres  converge 
to  be  inserted  into  the  triangular  area  on  the  ventral  surface  of  the 
scapula  near  its  cervical  angle,  in  common  with  the  anterior  fibres  of 
the  preceding  muscle,  from  which  it  is  not  distinct.  The  two  muscles, 
taken  together,  have  a  well-marked  f\\n-like  an-angement,  having  an 
extensive  convex  border  where  they  take  origin,  while  they  converge  to 
a  comparatively  narrow  point  at  their  insertion. 

Action. — The  levator  anguli  scapulae  carries  the  articular  angle  of  the 
scapula  backwards  by  pulling  the  cervical  angle  forwards ;  but  when 
the  scapula  is  fixed,  the  right  and  left  muscles,  acting  together,  can 
raise  the  cervical  portion  of  the  spinal  column,  or  the  single  muscle  can 
incline  it  to  one  side. 

The  Subscapulo-hyoid.  This  muscle,  which  arises  from  the  sub- 
scapular fascia,  is  described  with  the  dissection  of  the  neck. 

THE  OUTER  SCAPULAR  REGION. 

Position. — The  muscles  which  pass  between  the  shoulder  and  the 
trunk,  on  the  outer  aspect  of  the  former,  must  next  be  dissected ;  and, 
to  permit  this,  the  subject  must  be  placed  in  an  entirely  new  position. 
The  standing  posture  of  the  animal  is  the  best  for  this  purpose ;  and  it 
may  be  imitated  by  suspending  the  subject  to  a  stout  iron  rod  provided 
with  chains  and  hooks,  and  capable  of  being  raised  or  lowered  by  means 
of  a  system  of  pulleys  or  a  small  windlass. 

Surface-marhing. — About  the  centre  of  the  region  to  be  dissected  the 
student  will  feel  the  spine  of  the  scapula,  the  most  prominent  part  of 
which  is  its  tubercle.  In  a  well-nourished,  sound  horse  the  spine  should 
not  be  very  distinctly  visible,  but  in  an  emaciated  animal,  or  in  one 
whose  scapular  muscles  are  atrophied  as  an  accompaniment  of  joint- 
disease,  it  forms  a  very  prominent  ridge. 

Directions. — An  incision  through  the  skin  is  to  be  made  along  the 
spine  of  the  scapula  from  the  withei-s  to  the  middle  of  the  arm,  where 
a  transverse  incision  is  to  be  made  from  the  anterior  to  the  posterior 
border  of  the  limb.  Another  incision  is  to  be  carried  along  the  middle 
line  of  the  back,  and  prolonged  forwards  along  the  neck  by  the  dissector 
of  that  region,  and  backwards  to  the  lumbar  region  by  the  dissector  of 
the  back.  The  dissectors  of  the  three  regions  should  here  work 
together,  the  skin  being  turned  down  as  a  single  flap  from  the  neck  and 
anterior  half  of  the  shoulder,  and  as  another  flap  from  the  back  and 
posterior  half  of  the  shoulder.  The  skin,  it  will  be  observed,  is  thicker 
than  in  the  pectoral  region,  and  it  has  the  panniculus  carnosus  attached 
to  its  inner  surface.  Care  must  be  taken  not  to  remove  this  panniculus 
with  the  skin. 

The  Panniculus  Carnosus  is  the  muscle  which  enables  the  horse  to 


DISSECTION    OF    THE    ANTERIOR    LIMB.  » 

twitch  its  skin,  and  thus  remove  offending  insects.  It  is  most  extensive 
over  the  thorax  and  abdomen,  but  it  is  here  carried  over  the  muscles 
covering  the  scapula  and  humerus.  Before  the  muscle  passes  on  to 
the  limb,  it  sends  an  aponeurotic  layer  inwards  between  the  limb  and 
the  chest-wall.  At  its  upper  border  this  layer  is  provided  with  a  small 
tendon,  which  becomes  inserted  into  the  inner  tuberosity  of  the  humerus, 
and  which  will  be  seen  when  the  limb  is  dissected  from  the  trunk.  A 
nerve  will  be  seen  ramifying  in  the  scapulo-humeral  part  of  the  panni- 
culus.  This  turns  round  the  posterior  border  of  the  limb ;  and,  as 
already  seen,  it  is  formed  by  the  union  of  the  subcutaneous  thoracic 
with  some  perforating  intercostal  nerves. 

Directions. — The  panniculus  is  now  to  be  dissected  away  from  the 
limb ;  and  in  doing  this  in  front,  care  is  to  be  taken  of  the  thin 
cervical  trapezius  muscle,  which  might  be  mistaken  for  a  portion  of  the 
panniculus. 

The  Trapezius  in  the  horse  has  its  muscular  substance  interrupted 
by  a  tendinous  portion,  and  is  therefore  sometimes  described  as  two 
separate  muscles,  distinguished  as  the  cervical  and  the  dorsal  trapezius. 

The  Cervical  Trapezius  (Plate  4)  arises  from  the  funicular  por- 
tion of  the  ligamentum  nuchse ;  and  it  is  inserted  into  the  tubercle  on 
the  spine  of  the  scapula,  while  its  most  anterior  fibres  are  continuous 
with  an  aponeurosis  covering  the  scapular  muscles.  Both  the  deep  and 
the  superficial  face  of  the  muscle  have  a  thin,  adherent,  fibrous  covering, 
the  direction  of  whose  fibres  is  at  right  angles  to  that  of  the  muscular  fibres. 

Action. — It  draws  the  scapula  forwards  and  upwards. 

The  Dorsal  Trapezius  (Plate  4)  is  continuous  with  the  preced- 
ing by  the  aponeurotic  centre  already  mentioned.  It  arises  from  the 
summits  of  a  few  of  the  anterior  dorsal  spines,  and  is  inserted  into  the 
tubercle  on  the  scapular  spine. 

Action. — It  pulls  the  scapula  backwards  and  upwards. 

Directions. — Both  divisions  of  the  trapezius  are  now  to  be  severed 
close  to  their  origin,  and  reflected  downwards ;  and  while  this  is  being 
done,  search  is  to  be  made  for  the  branches  of  the  11th,  or  spinal 
accessory,  nerve,  which  enter  their  deep  face.  The  muscles  which  were 
covered,  wholly  or  in  part,  by  the  trapezius,  will  now  be  exposed. 
These  are  :  the  splenius,  the  levator  anguli  scapulae,  the  supraspinatus, 
the  infraspinatus,  the  anterior  deep  pectoral,  the  latissimus  dorsi,  and 
the  rhomboideus. 

It  will  be  remembered  that  in  the  dissection  of  the  pectoral  region 
the  anterior  deep  pectoral  could  not  be  followed  to  its  termination. 
The  reflected  portion  of  the  muscle  is  here  seen  (Plate  4),  but  is  partly 
covered  by  the  insertion  of  the  mastoido-humeralis. 

The  Latissimus  Dorsi  (Plate  4).  Though  neither  the  origin  nor 
the  insertion  of  the  muscle  is  found  here,  attention  should  be  given  to 


10  THE  ANATOMY  OF  THE  HORSE. 

it  as  it  is  being  exposed  by  the  dissector  of  the  back.  It  arises  bv  an 
aponenrotic  tendon  from  the  series  of  vertebral  spines,  beginning  about 
the  4th  dorsal,  and  extending  backwards  to  the  last  lumbar.  This 
tendon  is  succeeded  by  a  thick  muscular  portion,  which  contracts  and 
passes  in  between  the  limb  and  the  trunk,  where  it  will  afterwards  be 
followed  to  its  insertion  into  the  internal  tubercle  of  the  humerus.  Its 
anterior  fibres  will  be  noticed  to  play  over  the  dorsal  angle  and  cartilage 
of  prolongation  of  the  scapula. 

Action. — It  is  a  flexor  and  an  inward-rotator  of  the  shoulder-joint. 

The  Rhomboideus  (Plate  4),  like  the  ti-apezius,  comprises  a  cervical 
and  a  dorsal  portion.  The  ce^'vical  part  is  an  elongated,  narrow  muscle, 
which  extends  as  fixr  forward  as  the  axis,  and  arises  from  the  funicu- 
lar part  of  the  ligamentum  nucha).  Its  fibres  take  a  very  oblique 
direction  downwards  and  backwards,  and  are  inserted  into  the  anterior 
part  of  the  cartilage  of  prolongation  on  its  inner  surface,  being  there 
confounded  with  the  insertion  of  the  levator  anguli  scapulae.  The 
dorsal  portion  consists  of  fibres  which  a7-ise  from  the  anterior  dorsal 
spines,  and  pass  in  a  nearly  vertical  direction  to  be  insei-ted  into 
the  inner  surface  of  the  cartilage  of  prolongation,  behind  the  fibres  of 
the  cervical  division.  It  will  be  recollected  that  the  nerve  to  these 
muscles  passes  from  the  6th  cervical  nerve,  and  reaches  its  destination 
by  traversing  the  levator  anguli  scapula). 

Action. — To  pull  the  scapula  upwards  and  forwards  on  the  chest-wall. 

The  Mastoido-humeralis,  or  Levator  Humeri  (Plate  4).  This 
muscle,  in  the  greatest  part  of  its  extent,  is  found  in  the  head  and 
neck,  where  it  takes  its  wigin  from  the  mastoid  crest  and  the  trans- 
verse processes  of  the  first  four  cervical  vertebrae ;  but  attention  must 
here  be  given  to  its  insertion,  which  is  into  the  external  lip  of  the  musculo- 
spiral  groove,  after  covering  the  shoulder-joint.  It  receives  here  some 
branches  from  the  circumflex  nerve. 

Action. — It  is  an  extensor  and  inward-rotator  of  the  shoulder-joint. 
When  the  limb  is  fixed,  it  bends  the  neck  laterally. 

Directions. — The  limb  may  now  be  detached  from  the  trunk  by 
severing  the  attachment  of  the  rhomboideus,  serratus  magnus,  levator 
anguli  scapula),  mastoido-humeralis,  and  latissimus  dorsi,  the  last  being 
cut  where  it  plays  over  the  angle  of  the  scapula.  Pieces  of  clean  cloth 
saturated  with  some  preservative  solution  should  be  placed  on  the  outer 
aspect  of  the  shoulder  where  the  skin  has  been  removed,  while  the 
dissector  proceeds  to  examine  the  structures  over  the  inner  suiface  of 
the  scapula  and  humerus. 

inner  aspect  of  the  shoulder  and  arm. 
Directions. — The  dissector  should  now  identify  the  tcnninal  portions 
of  the  muscles  already  dissected,  and  cut  them  off  within  an  inch  or 


DISSECTION    OF    THE    ANTERIOR    LIMB.  .11 

two  of  their  insertion,  except  in  the  case  of  the  latissimus  dorsi,  which 
is  to  be  left  at  its  present  length  until  its  nerve  and  artery  have  been 
followed.  The  posterior  superficial  pectoral  should  be  cut  away  on  a 
level  with  the  olecranon,  but  care  is  to  be  taken  not  to  disturb  the 
vessels  and  nerves  which  it  covers.  The  aponeurosis  which  the 
panniculus  sends  within  the  shoulder  will  now  be  observed,  and,  at  its 
upper  border,  a  small  glistening  band  passing  to  be  inserted  into  the 
internal  tuberosity  of  the  humerus. 

The  next  step  is  to  dissect  out  the  axillary  and  brachial  vessels,  and 
the  remaining  branches  of  the  brachial  plexus ;  and  this  is  an  operation 
demanding  time  and  care.  While  an  assistant  holds  the  nerves  on  the 
stretch,  the  fat  and  areolar  connective-tissue  which  surround  them  and 
the  vessels,  are  to  be  cleaned  away  piecemeal,  always  proceeding  from 
the  main  trunks  to  the  branches.  In  doing  this,  the  dissector  will  meet 
two  groups  of  lymphatic  glands. 

Brachial  Lymphatic  Glands.  The  upper  group  consists  of  a  cluster 
placed  behind  the  brachial  vessels,  on  a  level  with  the  middle  of  the 
humerus.  The  lower  group  consists  of  one  or  two  glands  in  relation  to 
the  vessels,  just  above  the  elbow-joint. 

The  Axillary  Artery  (Plates  5  and  6).  This  vessel  has  already  been 
seen  passing  in  a  curved  direction  from  the  anterior  border  of  the  1st 
rib,  across  the  inner  aspect  of  the  shoulder-joint,  where  it  rests  above  the 
terminal  insertion  of  the  posterior  deep  pectoral,  and  on  the  tendon  of 
the  subscapularis.  It  passes  on  to  the  teres  major,  and  is  continued  as 
the  brachial  artery.  In  this  course  it  gives  off  four  vessels,  viz.,  the 
inferior  cervical,  e'xternal  thoracic,  suprascapular,  and  subscapular. 
The  first  two  have  already  been  dissected  in  the  axilla. 

The  Suprascapular  Artery  (Plate  5)  is  a  small,  tortuous  vessel 
springing  from  the  upper  surface  of  the  axillary  artery  about  the  middle 
of  its  extra-thoracic  course.  It  passes  upwards  for  a  short  distance, 
and  then  divides  into  branches,  the  longest  of  which  passes  over  the 
subscapularis  to  reach  the  anterior  deep  pectoral.  A  branch  passes 
in  between  the  subscapularis  and  the  supraspinatus,  while  smaller 
branches  ai-e  expended  in  the  tendons  about  the  shoulder. 

The  Subscapular  Artery  (Plates  5  and  6)  is  a  comparatively  large 
vessel,  and  beyond  its  origin  the  parent  trunk  is  much  reduced  in  calibre. 
It  arises  at  the  interstice  between  the  subscapularis  and  teres  major 
muscles ;  and,  disappearing  between  these  muscles,  it  ascends  behind 
the  glenoid  border  of  the  scapula,  as  far  as  its  dorsal  angle.  It  gives 
off  a  considerable  number  of  vessels  that  cannot  at  this  stage  be 
completely  followed,  but  near  its  origin  it  will  be  seen  to  throw  off  a 
branch  which  runs  upwards  and  backwards  on  the  latissimus  dorsi 
(Plate  5). 

The  Brachial  Artery  (Plates  5  and  6)  is  the  direct  continuation  of  the 


1-  THE  ANATOMY  OF  THE  HORSE. 

axillary,  which  changes  its  name  when  it  passes  on  to  the  teres  major. 
It  descends  in  a  nearly  vertical  direction  to  the  lower  extremity  of  the 
humerus,  where,  above  the  inner  condyle,  it  divides  to  form  the  anterior 
and  posterior  radial  arteries.*  In  its  course  it  crosses  the  direction  of 
the  Inmierus  obliquely,  and  rests  successively  on  the  tendons  of  the  teres 
major  and  latissimus  dorsi,  the  small  head  of  the  triceps,  and  the  bone. 
In  front  of  it  is  first  the  coraco-humeralis,  and  then  the  biceps  ;  but 
these  are  separated  from  it  by  the  median  nerve,  which  is  in  close 
contact  with  the  vessel.  Behind  the  artery  is  the  satellite  vein, 
posterior  to  which  is  the  ulnar  nerve.  Its  collateral  bi'anches  are  :  the 
pre-humeral,  the  deep  humeral,  the  ulnar,  the  nutrient  artery  of  the 
humerus  (sometimes),  and  innominate  muscidar  branches. 

The  Pre-humeral  or  Anterior  Circumflex  Artery  (Plate  6)  arises 
at  the  tendon  of  the  teres  major,  and  passes  in  front  of  the  humerus, 
between  the  upper  and  lower  insertions  of  the  coraco-humeralis,  to 
terminate  in  the  biceps  or  the  mastoido-humeralis.  Some  of  its  fine 
twigs  may  anastomose  with  divisions  of  the  posterior  circumflex. 

The  Deep  Humeral  Artery  (Plates  5  and  6)  arises  at  the  lower  border  of 
the  latissimus  dorsi  tendon,  and  soon  splits  into  three  or  four  branches, 
the  larger  of  which  perforate  the  large  head  of  the  triceps  extensor 
cubiti,  while  the  smaller  supply  the  small  and  mediiim  heads  of  the 
same  muscle.  A  branch  is  continued  round  behind  the  humerus,  in 
company  with  the  musculo-spiral  nerve,  to  the  front  of  the  elbow-joint, 
where  it  anastomoses  with  branches  of  the  anterior  radial.  This 
branch  will  not  be  followed  at  present. 

Muscular  Branches  of  the  Brachial.  The  largest  and  most  constant 
of  these  is  a  vessel  of  considerable  size  which  penetrates  the  lower  part 
of  the  biceps  (Plate  6). 

The  Ulnar  artery  and  the  two  terminal  branches  of  the  brachial  will 
be  followed  in  the  dissection  of  the  fore-arm. 

The  Brachial  Vein  is  a  large  vessel  which  ascends  behind  the 
artery,  and  receives  branches  that  for  the  most  part  correspond  to  those 
of  the  artery.      It  receives  also  the  subcutaneous  thoracic  or  spur  vein. 

Directions. — As  the  brachial  vein  generally  contains  a  large  quantity 
of  blood  which  exudes  from  the  smaller  cut  branches,  it  will  contribute 
to  the  neatness  and  cleanness  of  the  dissection  if  the  dissector  will 
carefully  remove  the  vein  and  all  its  branches  before  he  proceeds  to 
follow  the  nerves. 

The  Brachial  Plexus.  The  mode  of  formation  of  the  plexus  has 
already  been  explained,  and  the  student  will  recollect  that  he  has 
already  followed  branches  fi-om  it  to  the  levator  anguli  scapuhe, 
sen-atus  magnus,  and  pectoral  muscles,  as  well  as  the  subcutaneous 

'*  In  Plate  6  the  termination  of  the  bi-achial  artery  has  been  pulled  slightly  forwards  in  order  to 
bIiow  the  origin  of  the  anterior  radial  artery. 


DISSECTION    OP    THE    ANTERIOR    LIMB.  13 

thoracic  nerve,  and  the  filament  furnished  by  the  plexus  to  the  phrenic 
nerve.      He  can  now  easily  identify  and  trace  the  following  branches  : — 

The  Nerve  to  the  Latissimus  Dorsi  (Plate  5)  derives  its  fibres 
from  the  8th  cervical  and  the  dorsal  roots  of  the  plexus. 

The  Nerve  to  the  Teres  Major  (Plate  5) — one  or  more  filaments, 
generally  deriving  fibres,  in  common  with  the  circumflex  nerve,  from 
the  7th  and  8th  cervical  roots  (with  possibly  some  fibres  from  the  6th). 

The  Nerve  to  the  Subscapularis  (Plate  5)  derives  its  fibres  from 
all  the  cervical  roots  of  the  plexus. 

The  Circumflex  Nerve  (Plates  5  and  6).  Its  fibres  come  from  the  7th 
and  8th  cervical  roots,  and  possibly  also  from  the  6th.  It  turns  round 
behind  the  shoulder-joint  in  company  with  the  posterior  circumflex 
artery  ;  and  on  the  outside  of  the  joint  it  supplies  branches  to  the  teres 
minor,  deltoid,  mastoido-humeralis,  and  skin  (Plate  7).  It  gives  a  twig 
to  the  small  scapulo-humeral  muscle. 

The  Suprascapular  Nerve  (Plate  5),  deriving  its  fibres  from  the 
6th,  7th,  and  8th  cervical  roots,  passes  into  the  interstice  between  the 
subscapularis  and  the  supraspinatus.  It  then  turns  round  the  anterior 
border  of  the  scapvila  ;  and  gaining  its  dorsal  surface,  is  expended  in 
the  supraspinatus  and  subspinatus  muscles  (Plate  8). 

The  MuscuLO-SPiRAL  Nerve  (or  radial  nerve)  (Plates  5  and  6)  is,  at  its 
origin,  the  thickest  of  the  nerves  of  the  brachial  plexus.  Deriving  its  fibres 
from  the  7tli  and  8th  cervical,  and  from  the  dorsal  roots  of  the  plexus,  it 
passes  downwards  and  backwards  on  the  subscapularis  and  teres  major 
muscles,  and  some  little  distance  behind  the  axillary  vessels,  from 
which  it  is  separated  by  the  ulnar  nerve.  On  reaching  the  deep 
humeral  artery,  it  disappears  in  front  of  the  large  head  of  the  triceps, 
and  is  continued  round  the  humerus  in  the  musculo-spiral  groove, 
where  it  rests  on  the  brachialis  anticus  (humeralis  externus),  and,  after- 
wards, at  the  posterior  or  outer  border  of  that  muscle.  It  reaches  the 
front  of  the  elbow-joint,  being  here  deeply  placed  between  the  brachialis 
anticus  inw^ardly,  and  the  origin  of  the  great  extensor  of  the  metacarpus 
outwai'dly.  Before  the  nerve  disappears  behind  the  humerus,  it  gives 
branches  to  the  great  and  small  heads  of  the  triceps,  and  a  long  branch 
which  passes  backward  to  divide  under  the  scapulo-ulnaris  for  the 
supply  of  that  muscle.  Behind  the  limb  it  supplies  the  medium  head 
of  the  triceps  and  the  anconeus,  and  furnishes  a  few  cutaneous  branches 
which  perforate  the  caput  medium,  or  emerge  at  its  lower  part,  to  be 
distributed  to  the  skin  of  the  outer  side  of  the  fore-arm,  below  the 
elbow.  The  termination  of  the  nerve  will  afterwards  be  followed  in 
the  fore-arm,  where  it  supplies  the  extensor  muscles  and  the  flexor 
metacarpi  externus. 

The  Ulnar  Nerve  (Plates  5  and  6)  derives  its  fibres  from  the  dorsal 
roots  of  the  brachial  plexus.      At  first  it  lies  close  behind  the  main 


14  THE  ANATOMY  OF  THE  HORSE. 

vessels ;  but  as  it  passes  downwards,  it  recedes  from  them,  and  passing 
under  cover  of  the  scapulo-ulnaris,  it  reaches  the  space  between  the 
olecranon  and  the  inner  condyle.  Thence  it  descends  to  the  back  of  the 
fore-arm,  where  it  will  subsequently  be  dissected.  At  present  it  is  seen 
to  give  off  only  one  branch,  which  disappears  within  the  superficial 
pectoral  muscle,  and  afterwards  becomes  distributed  to  the  skin  of  the 
fore-arm  (Plate  5). 

The  ]\Iedian  Nerve  (Plates  5  and  6)  is  formed  by  the  union  of  two  roots. 
The  anterior  of  these  comes  from  the  6th,  7th,  and  8th  cervical,  while 
the  posterior  is  derived  from  the  8th  cervical  and  the  1st  dorsal.  These 
roots  gives  off  some  pectoral  twigs,  and  then  unite  by  forming  a  loop  in 
which  the  axillary  artery  rests.  The  nerve  then  descends  in  front  of 
the  axillary  ai'tery  and  its  brachial  continuation,  and  will  afterwards  be 
seen  to  accompany  the  posterior  radial  artery.  The  following  branches 
of  the  nerve  may  be  found  at  present : — 

The  Nerve  to  the  Biceps  and  Coraco-humeralis  comes  off  close  below  the 
union  of  the  two  roots  of  the  median,  or  from  the  anterior  root  above 
the  point  of  union.  It  passes  betw^een  the  upper  and  lower  insertions 
of  the  coraco-humeralis,  supplying  that  muscle  and  terminating  in  the 
biceps. 

Jftisculo-ciiianeous  branch. — This  is  given  off  from  the  median  about 
the  middle  of  the  humerus ;  and  passing  underneath  the  biceps,  it 
divides  into  a  muscular  branch  for  the  brachialis  anticus,  and  a  cutane- 
ous branch  for  the  front  of  the  fore-arm. 

Directions. — The  muscles  of  this  region  should  now  be  examined  in 
the  order  of  their  description. 

The  Latissimus  Dorsi  (Plates  5  and  6).  The  insertion  of  this  muscle 
into  the  inner  tubercle  of  the  humerus  is  here  seen.  About  an  inch  or 
two  from  its  termination  the  tendon  gets  a  twist  which  alters  the  direc- 
tion of  its  surfaces,  and  brings  it  to  be  inserted  in  front  of  tlie  termina- 
tion of  the  teres  major  on  the  same  tubercle. 

Action. — The  muscle  is  a  flexor  and  an  inward-rotator  of  the  shoulder- 
joint 

The  Teres  Major  (Plate  5).  It  ai-ises  from  the  dorsal  angle  of  the 
scapula,  and  from  an  aponeurosis  between  it  and  t'  subscapulars.  It 
is  inserted  into  the  internal  tubercle  of  the  humerus,  its  terminal  tendon 
resting  in  the  twist  formed  by  tlie  tendon  of  tlie  latissimus  dorsi  muscle. 

Action. — It  is  a  flexor  and  an  inward-rotator  of  the  shoulder. 

The  Scapulo-Ulnaris  (Plate  5).  This  is  a  thin,  flat  muscle  which 
rests  on  the  inner  surface  of  the  triceps,  and  is  provided,  in  front  and 
above,  with  a  thin,  transparent  tendon.  It  arises  from  the  posterior 
border  of  the  scapula,  and  is  inserted  into  the  posterior  border  of  the 
olecranon,  and  into  the  fascia  of  the  foi-e-arm.  At  its  lower  extremity 
the  muscle  covers  the  ulnar  vessels  and  nerves. 


DISSECTION    OF   THE    ANTERIOR    LIMB.  15 

Action. — To  extend  the  elbow-joiut,  and  tense  the  fascia  of  the  fore- 
arm. 

The  Triceps  Extensor  Cubiti.  This  is  an  immense  muscular  mass 
Avhich,  with  the  preceding,  fills  up  the  angle  formed  behind  the 
shoulder-joint.  It  has  three  divisions  or  heads,  which  may  be  distin- 
guished as  the  caput  magnum,  the  caput  medium,  and  the  caput 
parvum. 

The  Caput  Magnum,  or  large  head  (Plate  5),  forms  a  great  mass 
which  is  seen  on  both  the  outside  and  the  inside  of  the  limb.  It  arises 
from  the  dorsal  angle  and  glenoid  (posterior)  border  of  the  scapula ; 
and  it  is  inserted  into  the  olecranon,  there  being  a  synovial  bursa 
between  the  summit  of  that  eminence  and  the  tendon. 

The  Cajyut  Parvum,  or  small  head  (Plate  5),  is,  Avhen  compared  with 
the  preceding,  a  very  small  muscle.  It  arises  from  the  shaft  of  the 
humerus  below  and  behind  the  internal  tubercle,  and  it  is  inserted  into 
the  olecranon. 

The  Caput  Medium,  which  is  not  now  visible,  will  be  dissected  with 
the  outside  of  the  shoulder. 

Action  of  the  triceps.  It  is  an  extensor  of  the  elbow-joint,  and  acts 
as  a  lever  of  the  first  order,  the  joint,  which  represents  the  fulcrum, 
being  between  the  power  and  the  weight.  The  large  head  is  also  a 
flexor  of  the  shoulder. 

The  Subscapularis  (Plates  5  and  6).  This  muscle  is  lodged  in  the  fossa 
of  the  same  name  on  the  ventral  surface  of  the  scapula,  and  it  arises  from 
the  whole  extent  of  that  fossa.  It  is  inserted  into  the  inner  tuberosity 
of  the  humerus,  a  small  synovial  bursa  being  interposed  between  the 
tendon  and  the  bone.  The  tendon  is  crossed  by  the  origin  of  the 
coraco-humeralis,  and  another  small  bursa  is  here  interposed  between 
the  tendons.  Above  its  insertion  it  is  closely  related  to  the  capsular 
ligament  of  the  joint.  The  muscle  is  partly  united  in  front  with  the 
supraspiuatus,  and  behind  with  the  teres  major. 

Action. — It  is  an  adductor  of  the  shoulder. 

The  Coraco-humeralis  (or  coraco-brachialis)  (Plates  5  and  6).  This, 
which  is  rather  a  small  muscle,  arises  from  a  small  tubercle  on  the  inner 
side  of  the  coracoid  process  of  the  scapula.  It  has  two  insertions,  the  first 
into  the  inner  surface  of  the  shaft  of  the  humerus  above  the  internal 
tubercle,  the  second  into  a  line  which  begins  on  a  level  with  the 
internal  tubercle,  and  runs  down  the  anterior  surface  of  the  shaft  near 
its  inner  border.  Between  these  two  insertions,  the  pre-humeral  artery 
and  the  nerve  to  the  biceps  pass.  The  tendon  of  origin  of  the  muscle 
comes  out  between  the  supraspiuatus  and  subscapularis  muscles,  and 
the  posterior  border  of  the  muscle  is  related  to  the  brachial  artery. 

Action. — To  adduct  and  flex  the  shoulder. 

The  Biceps  (Plates  5  and  6).     This  muscle  receives  its  name  in  the 


IG  THE  ANATOMY  OF  THE  HORSE. 

human  subject  from  its  having  two  heads  of  origin.  It  is  also  known  as  the 
flexor  hrachii  or  coraco-radialis.  It  arises  from  the  whole  of  the  coracoid 
process  of  the  scapula  with  the  exception  of  the  tubercle  on  its  inner 
side,  which  is  for  the  coraco-humeralis.  Its  strong  tendon  of  origin 
emerges  from  between  the  outer  and  inner  tendons  of  the  supraspinatus, 
and  passes  over  the  shoulder-joint,  a  pad  of  fat  separating  its  deep  face 
from  the  capsular  ligament  of  the  joint.  The  tendon,  which  is  of  fibro- 
cartilaginous consistency,  then  plays  over  the  bicipital  gi'oove  of  the 
humerus,  on  which  its  deep  face  is  moulded,  and  a  synovial  bursa 
facilitates  the  movements  of  the  tendon  iu  the  groove.  The  central 
portion  of  the  muscle,  which  is  thick  and  fusiform,  has  niimerous 
tendinous  intersections,  and  is  traversed  throughout  by  a  fibrous  cord. 
It  rests  on  the  anterior  face  of  the  humerus,  and  at  its  lower  end 
terminates  by  a  tendon  which,  passing  over  the  anterior  ligament  of 
the  clbow-joiut  (to  which  it  is  adherent),  is  inserted  into  the  bicipital 
tuberosity  of  the  radius.  The  tendon  is  partly  covered  by  the  internal 
lateral  ligament  of  the  elbow.  The  muscle  has  a  second  insertion,  in 
the  shape  of  a  strong  fibrous  band,  detached  from  the  main  tendon  to 
blend  with  the  sheath  of  the  extensor  motacarpi  magnus,  and  deep 
fascia  on  the  front  of  the  fore-arm. 

Action. — To  flex  the  elbow-joint,  and  make  tense  the  f\\scia  of  the 
fore-arm.  In  the  first  of  these  actions  it  is  a  good  example  of  a  lever  of 
the  third  order,  where  the  power  is  applied  between  the  fulcrum — 
represented  by  the  elbow-joint,  and  the  weight — represented  by  the 
distal  portion  of  the  limb.  The  fibrous  cord  which  traverses  the 
muscle  is  a  mechanical  extensor  of  the  shoulder-joint,  as  long  as  the 
elbow  is  kept  extended  by  the  ti-iceps  extensor  cubiti. 

Directions. — The  teres  major  from  the  shoulder  upwards  should  now 
be  removed,  in  order  to  follow  more  thoroughly  the  course  of  the  sub- 
scapular artery  with  its  branches,  and  to  expose  the  small  scapulo- 
humeral muscle,  which  lies  on  the  capsular  ligament  behind  the  joint ; 
but  care  should  be  taken,  in  dissecting  the  tendons  in  the  neighbour- 
hood of  the  joint,  to  preserve  the  capsular  ligament  intact. 

The  Subscapular  Artery  (Plate  6)  springs  from  the  axillary  trunk 
at  the  interstice  between  the  subscapularis  and  teres  major  muscles, 
and  disappearing  from  view,  runs  upwards  at  the  posterior  border  of  the 
scapula.     It  gives  off  as  its  most  important  branches  : — 

1.  A  Muscular  branch  of  considerable  volume  which  passes  backwards 
and  upwards  on  the  deep  face  of  the  latissimus  dorsi. 

2.  The  Posterior  circumflex  artery,  which  turns  round  behind  the 
shoulder,  passing  through  a  triangular  space  bounded  by  the  teres  major, 
caput  magnum,  and  scapulo-humeralis  gracilis.  At  the  outer  side  of 
the  joint  (Plate  7)  it  appears  between  the  caput  magnum,  caput  medium, 
and  teres  minor,  and  is  covered  by  the  deltoid.     It  splits  into  branches 


DISSECTION    OF    THE   ANTERIOR    LIMB.  17 

which  are  distributed  to  these  muscles  and  the  supraspinatus  (Plate  7). 
It  is  accompanied  by  the  circumflex  nerve. 

3.  Other  branches  of  the  subscapular  are  as  follows  : — A  few  inches 
above  the  origin  of  the  posterior  circumflex,  a  vessel  is  detached  which 
passes  backwards,  and  divides  to  supply  the  caput  magnum.  A  number 
of  smaller  branches  come  off"  from  the  anterior  aspect  of  the  vessel,  and 
are  distributed  on  both  surfaces  of  the  scapula.  One  of  these  supplies 
the  nutrient  artery  of  the  scapula. 

The  ScAPULO-HUMERALis  GRACILIS  is  a  very  slender  muscle.  It  arises 
from  the  scapula  above  the  rim  of  its  glenoid  cavity ;  and  passing  over 
the  capsular  ligament  of  the  shoulder,  on  which  some  of  its  fibres  seem 
to  terminate,  it  insinuates  itself  between  the  fibres  of  the  brachialis 
anticus  (humeralis  externus),  and  is  inserted  into  the  posterior  surface  of 
the  shaft  of  the  humerus.  It  is  supplied  by  a  small  nerve  from  the 
circumflex. 

Action. — The  muscle  is  too  inconsiderable  in  size  to  exercise  any 
appreciable  action  on  the  joint  over  which  it  passes,  and,  probably,  its 
function  is  to  raise  the  capsular  ligament  and  prevent  its  injury  during 
flexion  of  the  joint. 


OUTER    ASPECT    OF    THE    SHOULDER   AND    ARM. 

Directions. — The  limb  is  now  to  be  turned  over,  and  the  muscles  and 
other  structures  on  the  outer  side  of  the  scapula  and  humerus  are 
to  be  dissected. 

Scapular  Fascia. — This  is  a  strong,  glistening,  fibrous  covering  which 
is  spread  over  the  muscles  on  the  dorsum  of  the  scapula,  aftbrding  by 
its  inner  surface  an  origin  to  many  of  their  fibres.  When  traced 
upwards,  it  is  seen  to  be  inserted  into  the  scapula  or  its  cartilage  of  pro- 
longation; while  before,  behind,  and  inferiorly,  it  becomes  less  fibrous, 
and  is  continuous  with  the  fascia  covering  the  muscles  on  the  inner 
surface  of  the  scapula  and  the  outer  aspect  of  the  arm.  It  furnishes 
septa  to  pass  between  the  subjacent  muscles,  and  it  is  adherent  to  the 
tubercle  on  the  scapular  spine.  If  an  attempt  be  made  to  dissect 
it  off"  these  mtiscles,  they  will  be  exposed  with  a  rough  surface, 
showing  that  they  there  take  origin  from  the  inner  aspect  of  the 
fascia. 

The  Deltoid  Muscle  (scapular  portion)  (Plates  4  and  7).  This  muscle 
Avas  by  Percivall  en-oneously  termed  the  teres  minor.  It  is  not  the 
homologue  of  either  of  the  teres  muscles  of  human  anatomy,  but  is,  most 
clearly,  the  representative  of  that  part  of  the  deltoid  muscle  which  in 
man  takes  origin  from  the  scapula.  A  linear  depression  Avhich  traverses 
the  muscle  corresponds  to  an  imperfect  division  of  it  into  an  anterior 
and  a  posterior  portion.      It  arises  by  its  anterior  poi-tion  from  the 

c 


18  THE  ANATOMY  OF  THE  HORSE. 

scai)ular  fascia,  and  by  its  posterior  portion  from  the  dorsal  angle  of 
the  scapula.  It  is  inserted  into  the  deltoid  (external)  tubercle  of  the 
humerus. 

Action. — To  abduct  the  humerus,  and  rotate  it  outwards.  Acting 
with  the  teres  major,  it  is  also  a  flexor  of  the  shoulder. 

Directions. — The  last-mentioned  muscle  should  be  carefully  cut  at 
the  level  of  the  shoulder,  and  reflected  upwards  and  downwards.  This 
will  expose  the  divisions  of  the  circumflex  vessels  and  nerve,  branches 
of  which  will  be  seen  entering  the  muscle,  and  it  will  at  the  same  time 
bring  into  view  the  next  muscle. 

The  Teres  Minor  (Plates  7  and  8).  |This  small  muscle  arises  from  the 
posterior  border  of  the  scapula,  from  the  rough  lines  at  the  lower  part 
of  the  infraspinous  fossa,  and  from  the  small  tubercle  on  the  outer  rim 
of  the  glenoid  cavity.  Its  tendon,  which  is  crossed  by  a  glistening 
band  of  fascia,  is  inserted  into  the  loAver  half  of  the  ridge  running 
upwards  from  the  deltoid  tubercle  to  the  extenial  tuberosity. 
Action. — The  same  as  the  preceding  muscle. 

The  Infraspinatus  (subspinatus,  or  postea-spinatus)  (Plates  7  and  8) 
occupies  the  greater  part  of  the  fossa  of  the  same  name.  It  aHses  from 
the  whole  extent  of  the  fossa,  and  from  the  inner  surface  of  the  scapular 
fascia.  It  possesses  two  tendons  of  insertion,  the  outer  of  which  passes 
over  the  convexity  of  the  external  tuberosity,  a  synovial  bursa  being 
interposed,  and  is  inserted  into  the  upper  half  of  the  ridge  connecting 
that  tuberosity  to  the  deltoid  tubercle.  If  this  tendon  be  cut  where  it 
plays  over  the  convexity,  the  synovial  bursa  will  be  opened,  and, 
at  the  same  time,  the  inner  insertion  of  the  muscle  into  the  inside 
of  the  convexity  will  be  exposed.  This  inner  tendon  is  more  fleshy 
than  the  outer,  and  is  in  contact  with  the  capsular  ligament  of  the 
shoulder. 

Action. — It  abducts  the  humerus,  and  rotates  it  outwards. 
The  SupRASPiNATUS  (antea-spinatus)  (Plates  7  and  8)  fills  the  whole  of 
the  fossa  of  the  same  name,  and  takes  ongin  from  it  as  well  as  from 
the  scapular  fascia.  It  is  bifid  inferiorly,  having  an  inner  tendon 
inse7'ted  into  the  internal  tuberosity  at  its  highest  point,  and  an  outer 
tendon  inserted  into  the  corresponding  point  of  the  external  tuberosity. 
These  two  tendons  are  in  contact  with  the  capsular  ligament  of  the 
joint,  and  the  tendon  of  origin  of  the  biceps  emerges  from  between  them. 
Action. — It  is  an  extensor  of  the  shoulder-joint. 

Directions. — The  outer  aspect  of  the  triceps  extensor  cubiti  is  here 
seen;  and  when  its  surface  has  been  cleaned,  a  line  will  be  observed 
running  from  the  shoulder  to  the  point  of  the  elbow.  Careful  dissection 
downwards  into  the  mass,  along  this  line,  will  separate  the  caput  mag- 
num (already  described)  from  the  caput  medium,  which  lies  below  it. 
"While  the  surface  of  the  muscle  is  being  cleaned,  some  small  cutaneous 


DISSECTION    OF    THE    ANTERIOR    LIMB.  19 

nerves  from  the  musculo -spiral  will  be  found  to  pierce  the  muscle,  or 
emerge  at  its  loAver  edge,  and  become  distributed  to  the  outer  side  of  the 
fore-arm.     These  should,  as  far  as  possible,  be  preserved. 

The  Caput  Medium  (Plates  7  and  8)  arises,  by  a  short  aponeurotic  ten- 
don, from  a  curved  line  beginning  on  the  deltoid  tubercle  and  continued 
upwards  to  the  external  tuberosity.     It  is  inserted  into  the  olecranon. 

Action. — ^Like  the  other  divisions  of  the  triceps,  this  muscle  is  an 
extensor  of  the  elbow-joint. 

Directions. — By  raising  the  lower  edge  of  the  last  muscle  and  dissect- 
ing upwards,  the  anconeus  will  be  partly  exposed ;  but  to  effect  a  com- 
plete and  natural  separation  of  the  two  muscles,  is  a  matter  of  some 
difficulty. 

The  Anconeus  (Plates  7  and  8)  is  a  small  muscle  which  lies  above  the 
olecranon  fossa,  and  there  covers  the  synovial  membrane  of  the  joint, 
a  pad  of  fat  being  interposed.  It  arises  from  the  margin  of  the  fossa, 
and  is  inserted  into  the  oleci'anon  on  its  outer  and  anterior  aspect. 

Action. — To  assist  in  extending  the  elbow,  and  at  the  same  time  to 
raise  the  synovial  membrane  and  prevent  its  injury  between  the  bones. 

Directions. — If  the  caput  medium  be  now  severed  at  its  origin,  and 
turned  backwards,  the  musculo-spiral  nerve  and  some  branches  of  the 
deep  humeral  artery  will,  as  already  described,  be  found  turning  round 
the  humerus  in  the  musculo-spiral  groove,  which  is  mainly  filled  by  the 
brachialis  anticus  muscle. 

The  Brachialis  Anticus  muscle  (Plate  8),  also  known  as  the  humeralis 
obliquus  or  externus,  is  lodged  in  the  fiu-row  of  torsion  on  the  shaft  of  the 
humerus.  The  muscle  has  its  origin  on  the  posterior  aspect  of  the  shaft 
of  the  humerus  below  its  articular  head.  Its  tendon,  which  cannot  be 
followed  at  present,  passes  in  front  of  the  elbow-joint,  and  is  afterwards 
reflected  under  the  internal  lateral  ligament  of  the  joint,  to  be  inserted 
into  the  radius  and  ulna. 

Action. — To  flex  the  elbow-joint. 

the  fore-arm. 
Surface-marking. — At, the  elbow-joint  the  olecranon  process  of  the 
ulna  is  distinctly  seen ;  but  the  shafts  of  the  bones  of  the  fore-arm  ai'e 
clothed  with  muscles,  except  at  the  lower  third  of  the  inner  border  of 
the  radius,  where  the  bone  is  subcutaneous.  On  the  outer  side  of  the 
front  of  the  elbow-joint  a  large  muscular  mass  is  formed  by  the  extensor 
metacarpi  magnus  and  the  anterior  extensor  of  the  digit  (extensor 
pedis).  In  the  living  animal  (in  which  it  is  preferable  to  study  these 
surface-markings)  this  is  more  distinctly  visible,  and  the  tendons  of  these 
muscles  and  that  of  the  lateral  extensor  (extensor  suffi-aginis)  may  be 
distinctly  traced.  On  the  inner  side  of  the  elbow-joint  one  may  feel  the 
tendon  of  insertion  of  the  biceps;  and  just  behind  the  tendon  the  posterior 


L'U  THE  ANATOMY  OF  THE  HORSE. 

1-adial  vessels  and  the  median  nerve  may  be  felt  as  they  lie  on  the  bone 
under  cover  of  the  posterior  supei-ficial  pectoral,  and  they  may  be  made 
to  roll  under  the  finger.  This  should  be  practised,  as  the  posterior 
radial  artery  is  a  convenient  vessel  at  which  to  feel  the  pulse.  The 
internal  subcutaneous  vein  crosses  the  inner  ftice  of  the  fore -arm 
obliquely  upwards  and  forwards ;  and  in  the  living  animal,  pressure  at 
the  upper  part  Avill  distend  the  vessel  and  bring  it  into  view.  At  the 
outer  side  of  the  carpus  the  prominence  foi*med  by  the  pisiform  bone 
may  be  seen  and  felt.  On  the  inner  surface  of  the  fore-arm,  at  its  lower 
third,  the  skin  presents  an  oval-shaped,  horny  callosity,  vulgarly  termed 
the  chestnut.     This  is  largest  in  coarse-bred  animals. 

Directions. — The  skin  is  now  to  be  carefully  removed  from  the 
fore -arm  and  carpus,  and  the  cutaneous  nerves  and  vessels  are  to  be 
sought. 

Cutaneous  Nerves.  (1)  At  the  front  of  the  elbow -joint  (Plate  8)  the 
cutaneous  division  of  the  musculo-cutaneous  branch  of  the  median  appears 
from  beneath  the  biceps,  and  splits  into  two  branches,  one  accompanying 
the  anterior,  the  other  the  internal,  subcutaneous  vein ;  (2)  a  little  way 
below  the  elbow,  on  its  inner  aspect,  the  cutaneous  branch  of  the  ulnar 
(Plate  5)  appears  from  beneath  the  insertion  of  the  posterior  superficial 
pectoral,  and  divides  for  the  supply  of  the  skin  of  the  back  of  the  fore-arm 
on  both  its  outer  and  its  inner  side ;  (3)  perforating  the  caput  medium, 
or  emerging  at  its  lower  edge,  are  some  twigs  from  the  musculo-spiral 
nerve,  which  are  distributed  to  the  skin  of  the  outer  side  of  the  fore-arm 
beneath  the  elbow ;  (4)  on  the  outer  side  of  the  carpus  (Plate  8)  are  the 
ramifications  of  a  cutaneous  branch  of  the  ulnar,  which  comes  out  be- 
tween the  tendons  of  the  external  and  oblique  flexors  of  the  metacarpus. 

Subcutaneous  Veins. — 1.  The  Median  or  Internal  subcutaneous  vein 
begins  at  the  inner  side  of  the  carpus,  where  it  continues  upwards  the 
internal  metacarpal  vein.  It  crosses  the  fore -arm  obliquely  upwards 
and  forwards,  in  company  with  a  cutaneous  nei've  already  described, 
and  divides  into  the  cephalic  and  basilic  veins.  The  Cephalic  vein  has 
already  been  seen  ascending  in  the  groove  between  the  mastoido- 
humeralis  and  the  anterior  superficial  pectoral  to  terminate  in  the 
jugular.  The  Basilic  vein  pierces  the  posterior  superficial  pectoral 
to  concur  in  forming  the  brachial  vein. 

2.  The  Anterior  subcutaneous  or  radial  vein  is  much  smaller  than  the 
preceding  vessel.  It  begins  at  the  front  of  the  carpus,  and,  ascending 
on  the  middle  line  of  the  fore-arm,  it  empties  itself  into  the  cephalic  or 
the  median  vein. 

Directions. — The  thin  superficial  fascia  in  which  these  nerves  and 
vessels  are  distributed  should  be  removed  to  show  the  deep  fascia. 

Deep  Fascia  of  the  fore-arm. — This  is  spread  in  the  form  of  a  close- 
fitting  fibrous  envelope  around  the  fore -arm.       Above  it   receives  an 


DISSECTION    OF    THE   ANTERIOR   LIMB.  21 

insertion  from  the  biceps,  and  another  from  the  scapulo-ulnaris ;  below 
it  is  continued  over  the  carpus  to  form  sheaths  for  the  tendons ;  while 
by  its  deep  face  it  furnishes  septa  to  pass  between  the  muscles  of  the 
fore-arm. 

Directions.— The  dissection  of  the  back  of  the  fore-arm  is  now  to  be 
undertaken.  The  before -mentioned  fascia  is  to  be  incised  along  the 
lines  of  separation  of  the  muscles,  and  these  are  to  be  cleaned  and 
isolated.  The  remaining  portion  of  the  posterior  superficial  pectoral 
muscle,  which  covers  the  posterior  radial  vessels  and  the  median  nerve 
at  the  inner  side  of  the  elboAv,  is  to  be  removed ;  and  care  is  to  be 
taken  of  the  ulnar  vessels  and  nerve,  which  are  placed  beneath  the 
deep  fascia,  on  the  middle  line  at  the  back  of  the  limb. 

The  Ulnar  Artery  (Plates  6  and  7)  is  a  collateral  branch  of  the  brachial, 
from  which  it  comes  off  at  the  lower  border  of  the  caput  parvum.  It 
descends  parallel  to  the  lower  border  of  that  muscle,  to  the  space  between 
the  olecranon  and  the  inner  condyle,  where  it  is  covered  by  the  scapulo- 
ulnaris.  It  here  places  itself  in  company  with  the  ulnar  nerve  ;  and, 
crossing  beneath  the  ulnar  origin  of  the  middle  flexor  of  the  metacarpus, 
it  descends  to  the  carpus  by  following  the  tendon  of  the  ulnar  portion  of 
the  deep  flexor  (ulnaris  accessorius),  being  placed  between  the  external 
and  oblique  flexors  of  the  metacarpus.  At  the  upper  limit  of  the  carpus 
it  concurs  in  the  formation  of  the  supracarpal  arch,  by  joining  a  branch 
detached  from  the  large  metacarpal  artery.  In  this  course  it  gives  oft" 
— (1)  the  nutrient  artery  to  the  humerus  (sometimes);  (2)  articular 
branches  to  the  elbow-joint ;  (3)  muscular  branches  in  the  neighbour- 
hood of  the  joint,  to  the  scapulo-ulnaris,  caput  parvum,  and  posterior 
superficial  pectoral ;  (4)  cutaneous  branches  to  the  skin  on  the  inner 
side  of  the  fore-arm. 

The  Ulnar  Vein  accompanies  the  ai-tery  and  nerve,  and  at  the  elbow 
concurs  in  the  formation  of  the  brachial  vein. 

The  Ulnar  Nerve  (Plates  6  and  8)  has  already  been  partly  described  in 
the  dissection  of  the  arm.  At  the  lower  part  of  that  region  it  crosses 
the  ulnar  artery,  with  which  it  places  itself  in  company  between  the  ole- 
cranon and  the  inner  condyle.  It  here  gives  off  branches  to  the  following- 
muscles  : — (1)  the  anterior  head  of  the  middle  flexor  of  the  metacarpus  ; 
(2)  the  ulnar  head  of  the  same  muscle ;  (3)  the  superficial  flexor  of  the 
digit  (perforatus) ;  (4)  the  ulnar  origin  of  the  deep  flexor  (ulnaris 
accessorius).  In  the  fore-arm  it  descends  in  close  company  with  the 
vessels  of  the  same  name,  and  at  the  carpus  it  gives  off  the  cutaneous 
branch  already  described  (page  20).  At  the  upper  border  of  the  pisiform 
bone,  and  beneath  the  tendon  of  the  middle  flexor,*  it  joins  a  branch 
from  the  median  to  form  the  external  plantar  nerve. 

*  In  Plates  6  and  9  the  termination  of  the  nerve  hais  been  i)ulled  slightly  forwards  to  show  its 
junction  with  the  branch  from  the  median. 


22  THE  ANATOMY  OF  THE  HORSE. 

The  Posterior  Radial  Artery  (Plate  6)  is  one  of  the  terminal 
branches  of  the  brachial.  It  is  so  much  larger  than  the  other  tenninal 
branch  (the  anterior  radial),  that  it  might  be  described  as  the  direct 
continuation  of  the  brachial,  whose  direction  it  prolongs.  Beginning 
above  the  inner  condyle,  it  descends  on  the  bone,  and  then  lies  over  the 
internal  lateral  ligament  of  the  elbow-joint,  and  posterior  to  the  tendon 
of  insertion  of  the  biceps.  It  is  here  covered  by  the  posterior  superficial 
pectoral,  and  is  related  to  the  median  nerve,  which  lies  close  behind  it, 
and  to  its  satellite  veins.  At  this  point  it  is  favourably  placed  for  taking 
the  pulse,  and  its  situation  and  relations  should  be  carefully  noted. 
After  crossing  the  elbow,  it  inclines  forwards  and  disappears  with  the 
median  nerve  between  the  radius  and  the  internal  flexor  of  the  meta- 
carpus. In  this  position  it  descends  to  within  a  short  distance  of  the 
carpus,  where  it  divides  into  two  terminal  branches  of  unequal  size — 
the  large  and  small  metacarpal  arteries.  It  gives  off  the  following 
collateral  branches : — 

1.  Articular  Branches  to  the  elbow-joint. 

2.  The  Interonseous  Artery  of  the  fore-arm,  which  reaches  the  outside  of 
the  limb  by  passing  through  the  radio-ulnar  arch.  It  then  descends  along 
the  outer  side  of  the  line  of  junction  of  the  radius  and  ulna  (Plate  7), 
where  it  Avill  be  followed  in  the  dissection  of  the  front  of  the  fore-arm. 

3.  Muscular  Branches  to  the  flexors  of  the  metacarpus  and  digit. 

4.  Cutaneous  Branches. 

The  Posterior  Radial  Veins.  Tlie  artery  is  accompanied  by  three  or 
four  satellite  veins,  which  surround  it  and  the  nerve,  and  anastomose 
freely  Avith  each  other.  They  begin  at  the  carpus,  where  they  anasto- 
mose with  the  metacarpal  veins,  and  at  the  elbow-joint  they  unite  with 
the  basilic  and  ulnar  veins  to  form  the  brachial  vein.  They  receive 
branches  corresponding  more  or  less  exactly  to  those  of  the  artery. 

The  Median  Nerve  in  the  fore-arm  (Plate  6).  This  nerve  has  already 
been  followed  in  the  dissection  of  the  arm,  where  it  was  seen  descending 
in  front  of  the  brachial  artery.  It  preserves  the  same  relationship  to  the 
first  few  inches  of  the  posterior  radial  artery,  but  at  the  elbow  it  crosses 
the  artery  superficially  to  take  up  a  posterior  position.  Below  the  joint 
it  again  changes  its  position  by  mounting  on  the  surface  of  the  artery, 
or  it  may  even  again  place  itself  in  front.  At  a  variable  point  in  the 
fore-arm  it  terminates  by  dividing  into  two  branches,  one  of  which  is 
continued  as  the  internal  plantar  nerve,  while  the  other  joins  the  ulnar 
to  form  the  external  plantar.  In  the  subject  from  which  Plate  6  was 
taken,  the  division  took  place  considerably  above  the  middle  of  the  fore- 
arm, but  more  frequently  it  occurs  in  the  lower  third.  Immediately 
below  the  elbow  the  nerve  furnishes  a  branch  to  the  internal  flexor  of 
the  metacarpus,  and  branches  to  the  deej)  flexor  of  the  digit  (humeral 
and  radial  heads). 


DISSECTION    OF    THE    ANTERIOR    LBIB.  23 

Directions. — The  muscles  on  the  back  of  the  fore -arm  must  now  be 
learnt.  These  consist  of  the  three  flexors  of  the  metacarpus,  and  the  two 
flexors  of  the  digit. 

The  Flexor  Metacarpi  Internus  (Plate  6).  This  muscle  lies  along 
the  inner  edge  of  the  posterior  surface  of  the  radius,  where  it  conceals 
the  posterior  radial  vessels  and  the  median  nerve.  It  arises  from  the 
inner  condyle  of  the  humerus,  just  behind  the  point  of  origin  of  the 
internal  lateral  ligament,  where  it  is  confounded  with  the  origin  of  the 
middle  flexor.  It  terminates  inferiorly  in  a  long,  slender  tendon,  which, 
after  passing  through  a  synovial  sheath  at  the  inner  side  of  the  carpus, 
is  inserted  into  the  head  of  the  inner  small  metacarpal  bone. 

Action. — It  is  a  flexor  at  the  carpal  articulations — i.e.,  it  flexes  the 
manus  on  the  fore-arm. 

The  Flexor  Metacarpi  Medius  (Plate  6).  This  muscle  descends  in 
contact  with  the  posterior  edge  of  the  internal  flexor.  It  has  two  heads 
of  origin — an  anterior  and  a  posterior.  It  arises  by  its  anterior  head 
just  behind  the  origin  of  the  preceding  muscle,  and  by  its  posterior 
head  from  the  upper  part  of  the  posterior  edge  of  the  olecranon.  After 
a  course  of  three  or  four  inches  these  two  heads  unite,  and  the  single 
inferior  tendon  is  inserted  into  the  upper  border  of  the  pisiform  bone. 
The  ulnar  nerve  and  vessels  pass  beneath  the  posterior  or  ulnar  head  of 
the  muscle. 

Action. — The  same  as  the  preceding  muscle. 

The  Flexor  Metacarpi  Extbrnus  (Plates  7  and  8)  is  situated  at  the 
outer  side  of  the  back  of  the  fore-arm,  having  the  lateral  extensor  of  the 
digit  (extensor  suffraginis)  in  front  of  it,  while  behind  it  is  separated  from 
the  last-described  muscle  by  the  ulnar  division  of  the  deep  flexor  of  the 
digit  (ulnaris  accessorius).  It  arises  from  the  lowest  point  of  the  outer 
I'idge  bounding  the  olecranon  fossa.  At  its  lower  end  it  has  two  inser- 
tions, viz.,  (1)  into  the  upper  border  of  the  pisiform  bone,  where  it  is 
confounded  with  the  insertion  of  the  middle  flexor ;  (2)  by  a  cord-like 
tendon  which,  after  descending  in  a  synovial  sheath  formed  inwardly  by 
the  oblique  groove  on  the  outer  surface  of  the  pisiform  bone,  is  inserted 
into  the  head  of  the  external  small  metacarpal  bone. 
Action.— LWe  the  preceding  two  muscles. 

Directions. — The  three  flexors  of  the  metacarpus  surrotmd  the  flexors 
of  the  digit,  and  they  should  be  cut  about  their  middle  and  reflected  to 
bring  these  latter  into  view. 

The  Superficial  Flexor  of  the  Digit  (flexor  pedis  perforatus)  (Plate 
6)  arises,  by  a  tendon  common  to  it  and  the  deep  flexor,  from  the 
lower  extremity  of  the  ridge  bounding  the  olecranon  fossa  on  the  inside. 
Its  muscular  belly  contains  much  tendinous  tissue,  and  cannot  without 
difficulty  be  separated  from  the  deep  flexor,  on  which  it  rests.  At  the 
lower  part  of  the  radius  its  muscular  portion  is  succeeded  by  a  tendon, 


24  THE  ANATOMY  OF  THE  HORSE. 

which,  after  being  reinforced  by  a  fibrous  band  from  the  back  of  the 
radius,  passes  through  the  carpal  sheath  behind  the  carpus,  and  is 
ultimately  inserted  by  a  bifid  tendon  into  the  second  phalanx.  Tlie 
examination  of  this  and  the  succeeding  muscle,  from  the  carpus  down 
wards,  must  be  postponed  till  the  dissection  of  the  metacarpus  and 
digit  is  luidertaken. 

Action. — The  muscle  flexes  successively  the  pastern,  fetlock,  and 
cai-pal  joints. 

The  Deep  Flexor  of  the  Digit  (flexor  pedis  perforans)  (Plate  6). 
This  muscle  is  situated  in  contact  with  the  posterior  surface  of  the 
radius,  and  consists  of  three  divisions,  which  may  be  distinguished  as 
the  humeral,  the  radial,  and  the  ulnar  portions.  The  humeral  or  main 
division  arises,  in  common  with  the  preceding  muscle,  from  the  lower 
extremity  of  the  ridge  bounding  the  olecranon  fossa  on  the  inside.  The 
radial  portion,  or  radialis  accessorius,  is  deeply  placed,  and  arises  from 
the  back  of  the  radius.  The  ulnar  division,  or  ulnans  accessomus,  is 
placed  beneath  the  deep  fascia  of  the  fore-arm,  whei-e  it  lies  between  the 
external  and  oblique  flexors  of  the  metacarpus,  and  is  accompanied  by 
the  ulnar  nerve  and  vessels.  It  arises  from  the  summit  and  posterior 
border  of  the  olecranon.  These  three  divisions  unite  above  the  carpus, 
and  have  a  common  tendon  which  passes  through  the  carjjal  sheath, 
and  is  ultimately  inserted  into  the  os  pedis. 

Action. — It  flexes  successively  from  below  upwards  the  inter-phalan- 
geal  joints,  the  fetlock,  and  the  carpus. 

Directions. — The  front  of  the  fore-arm  must  now  be  dissected  ;  and 
here  it  will  be  convenient  to  turn  attention  in  the  first  place  to  muscles ; 
but  while  these  are  being  isolated,  care  is  to  be  taken  of  the  interosseous 
vessels,  which  descend  along  the  lateral  extensor  at  the  outer  side  of  the 
region,  and  of  the  tendon  of  the  oblique  extensor  where  it  crosses  over 
the  tendon  of  the  extensor  metacarpi  magnus  above  the  carpus. 

The  ExTEXsoR  Metacarpi  j\Iagnus  (Plates  7  and  8)  corresponds  to  the 
long  and  short  radial  extensors  of  the  wrist  in  the  human  subject.  It  is 
a  powerful  muscle,  having  at  its  upper  end  a  massive  miiscular  belly, 
which  tapers  downwards,  and  terminates  a  few  inches  above  the  carpus 
in  a  tendon.  It  arises  from  the  anterior  and  upper  part  of  the  outer 
ridge  of  the  olecranon  fossa  (the  outer  condyloid  ridge),  where  this  ridge 
bounds  the  musculo-spiral  groove  ;  and  by  a  second  tendon,  in  common 
with  the  extensor  pedis,  from  a  depression  which  is  placed  external  to 
the  coronoid  fossa.  Its  inferior  tendon  lies  in  the  largest  and  most 
internal  of  the  vertical  grooves  at  the  lower  end  of  the  radius ;  and  after 
gliding  over  tlie  front  of  the  cai*pus  in  a  synovial  sheath,  it  is  inserted 
into  a  special  tubercle  on  the  upper  end  of  the  large  metacarpal  bone  at 
its  inner  side. 

Action. — It  extends  the  manus  on  the  fore-arm. 


DISSECTION    OF    THE    ANTERIOR    LIMB.  25 

The  Extensor  Mbtacarpi  Obliquus  {Plates  8  and  9).  This  is  the  re- 
presentative of  the  extensor  muscles  of  the  thumb  in  man.  It  a7'ises  from 
the  outer  side  of  the  radius ;  and  its  tendon,  after  passing  obliquely 
downwards  and  inwards  over  that  of  the  great  extensor,  is  inserted  into 
the  head  of  the  inner  small  metacarpal  bone.  It  lies  in  an  oblique 
groove  at  the  lower  end  of  the  radius,  Avhere  the  play  of  its  tendon 
is  facilitated  by  a  small  synovial  bursa. 

Action. — Like  the  preceding  muscle. 

The  Extensor  Pedis,  or  anterior  extensor  of  the  digit  (Plate  7), 
repi'esents  the  extensor  communis  digitorum  of  man.  At  its  origin  it 
lies  immediately  to  the  outer  side  of  the  extensor  metacarpi  magnus,  but 
at  the  lower  part  of  the  fore-arm  the  extensor  metacarpi  obliquus  emerges 
from  between  the  two  muscles.  It  arises,  by  a  tendon  common  to  it 
and  the  extensor  metacarpi  magnus,  from  a  depression  external  to  the 
coronoid  fossa ;  also  from  the  external  lateral  ligament  of  the  elbow,  and 
the  external  tuberosity  at  the  upper  end  of  the  radius.  It  consists  of 
two  parallel  portions  of  unequal  size,  and  these  are  succeeded  by  two 
tendons  wdiich  lie  close  together,  but  are  distinct  from  each  other. 
These  tendons  pass  in  common  through  a  vertical  groove  at  the  lower 
end  of  the  radius,  and  over  the  front  of  the  carpus,  where  they  are  pro- 
vided with  a  synovial  sheath.  In  the  dissection  of  the  metacarpus  and 
digit,  the  tendons  will  be  pursued  to  their  insertion,  the  outer  and 
smaller  *  joining  the  tendon  of  the  extensor  suffraginis,  while  the  inner 
and  main  tendon  becomes  inserted  into  the  pyramidal  process  of  the  os 
pedis. 

Action. — This  muscle  extends  in  succession  the  interphalangeal  joints, 
the  fetlock,  and  the  carpus. 

The  Extensor  Suffraginis,  or  lateral  extensor  of  the  digit  (Plates  7 
and  8),  is  a  smaller  muscle  than  the  extensor  pedis,  to  the  outer  side  of 
which  it  lies.  It  is  the  homologue  of  the  extensor  of  the  little  finger  in 
man.  It  arises  from  the  external  lateral  ligament  of  the  elbow,  from  the 
external  tuberosity  at  the  upper  end  of  the  radius,  from  the  line  of 
junction  of  the  radius  and  ulna,  and  from  the  outer  border  of  the  radius. 
Its  tendon  passes  first  through  a  vertical  groove  on  the  external  tuber- 
osity at  the  lower  end  of  the  radius,  then  through  a  synovial  sheath  at 
the  outer  side  of  the  carpus,  and  it  Avill  subsequently  be  followed  to  its 
insertion  into  the  first  phalanx. 

Action. — It  is  an  extensor  of  the  fetlock  and  of  the  cax'jjus. 

Directions. — The  nerves  and  bloodvessels  on  the  front  of  the  fore-arm 
must  next  be  sought,  and  in  order  to  fully  expose  them,  some  of  the 
foregoing  muscles  must  be  cut.      The  biceps  is  to  be  cut  about  its 

'  This  is  sometimes  termed  the  mv.scle  of  Phillips.  Occasionally  there  occurs,  to  the  inner  side 
of  the  preceding,  another  and  smaller  fasciculus,  with  a  slender  tendon  which  joins  the  main  tendon 
before  reaching  the  carpus.     This  is  the  muscle  of  Thieniessc. 


26  THE    ANATOMY    OF    THE    HORSE. 

middle  in  order  to  follow  the  anterior  radial  artex'j  ;  and  by  dissecting 
deeply  down  in  front  of  the  ^Ibow,  between  the  brachialis  anticus  and 
the  extensor  metacarpi  niagniis,  the  artery  will  be  found  to  meet  the 
musculo -spiral  nerve.  The  extensor  metacarpi  magnus  is  to  be  cut 
about  its  middle  and  carefully  reflected  in  order  to  follow  the  artery, 
which  lies  in  relation  to  the  deep  face  of  the  muscle ;  and  the  extensor 
pedis  is  to  be  similarly  reflected  to  trace  the  termination  of  the  mus- 
culo-spiral  nerve. 

The  Anteriou  Radial  Artery  (Plate  8)  is  the  smaller  terminal 
branch  of  the  brachial.  It  separates  at  an  acute  angle  from  the  pos- 
terior radial,  and  passes  forwards  beneath  the  biceps  and  then  beneath 
the  brachialis  anticus.  It  meets  the  musculo-spiral  nerve  in  the  inter- 
space between  the  brachialis  anticus  and  the  extensor  metacarpi  magnus, 
and  afterwards  descends  on  the  anterior  surface  of  the  radius,  where  it 
is  covered  by  the  last-mentioned  muscle.  It  terminates  at  the  carpus 
by  anastomosing  inwardly  with  branches  from  the  posterior  radial,  and 
outwardly  with  the  interosseous  artery  of  the  fore- arm.  It  supplies 
articular  branches  to  the  elbow,  and  muscular  branches  to  the  muscles 
on  the  front  of  the  fore-arm. 

The  Interosseous  Artery  of  the  fore-arm  (Plate  7)  is  a  branch 
given  off"  by  the  median  at  the  back  of  the  fore -arm.  It  comes  out- 
wards through  the  radio -ulnar  arch,  and  descends  along  the  extensor 
suffraginis,  terminating  in  slender  branches  in  front  of  the  carpus. 
It  supplies  articular  branches  to  the  elbow ;  the  nutrient  artery  of  the 
radius ;  and  miiscular  twigs  to  the  extensor  suffraginis,  extensor  pedis, 
and  extensor  metacarpi  obliquus. 

The  anterior  radial  and  interosseous  arteries  are,  generally,  compara- 
tively slender  vessels,  but  they  are  liable  to  some  variation  in  size  and 
distribution,  and  the  one  may  partly  supplant  the  other. 

Veins.  Satellite  veins  of  the  same  names  run  in  company  with  the 
foregoing  arteries. 

The  MuscuLO-spiUAL  Nerve  in  the  fore-arm  (Plate  8).  In  the  dissec- 
tion of  the  axilla  and  arm,  this  nerve  has  already  been  seen  as  a  large 
trunk  descending  fi-om  the  brachial  plexus,  and  taking  a  spiral  course  be- 
hind the  humerus.  It  reaches  the  front  of  the  elbow,  where  it  meets  the 
radial  artery  in  the  interspace  between  the  brachialis  anticus  inwardly, 
and  the  origin  of  the  extensor  metacarpi  magnus  outwardly.  It  here 
gives  off"  branches  to  the  extensor  metacarpi  magnus,  extensor  pedis, 
extensor  suff"raginis,  and  flexor  metacarpi  externus ;  and,  much 
reduced  in  size,  it  descends  between  the  shaft  of  the  radius  and  the 
extensor  pedis,  and  terminates  in  the  extensor  metacarpi  obliquus.  The 
nerve  to  the  flexor  metacarpi  externus  is  furnished  after  the  branches  to 
the  extensor  pedis,  and  passing  outwards  between  the  latter  muscle  and 
the  bone,  it  penetrates  its  muscle  at  the  radio-ulnar  arch. 


DISSECTION    OF    THE    ANTERIOR    LIMB.  J/ 

Directions. — In  this  stage  of  the  dissection  the  student  will  be  better 
able  to  trace  the  musculo-cutaneous  branch  of  the  median  nerve,  and  the 
insertions  of  the  biceps  and  brachialis  anticus  muscles  (see  pages  1 6  and 
1 9).  When  these  have  been  examined,  he  may,  as  the  next  step,  either 
dissect  the  articulations  of  the  shoulder  and  elbow  (pages  41  and  43),  or 
he  may  saturate  the  parts  already  dissected  with  some  preservative  solu- 
tion, and  postpone  the  examination  of  these  joints  till  after  the  dissec- 
tion of  the  metacarpus  and  digit. 


THE    METACARPUS    AND    DIGIT. 

The  distal  portion  of  the  horse's  fore  limb,  beyond  the  lower  extremity 
of  the  radius,  is  technically  termed  the  manus,  as  it  corresponds  to  the 
hand  of  man.  The  carpus,  or,  as  it  is  commonly  but  erroneously 
termed,  the  knee,  of  the  horse  corresponds  to  the  wrist  of  the  human 
subject.  The  portion  of  the  limb  between  the  carpus  and  the  fetlock, 
representing  the  palmar  portion  of  man's  hand,  is  called  the  metacarpus  ; 
while  the  rest  of  the  limb,  beyond  the  fetlock,  is  the  digit,  and  is  the 
homologue  of  man's  middle-finger. 

Surface-marking. — By  flexing  the  carpal  and  fetlock  joints,  the  sjdint 
bones  may  be  felt  at  the  back  of  the  metacarpus.  Behind  the  bones  in 
the  same  region  lie  the  flexor  tendons,  the  subcarpal  ligament,  and  the 
suspensory  ligament.  These,  whose  edges  may  be  more  or  less  distinctly 
seen  in  a  well-bred  animal,  have  the  relation  to  each  other  shown  in 
Plate  7.  Behind  the  fetlock-joint  is  a  tuft  of  hair  in  which  will  be 
found  a  horny  spur  or  ergot.,  which  is  largest  in  coarse-bred  animals. 
By  manipulation,  the  flexible  lateral  cartilages  may  be  felt  above  the 
hoof,  in  the  region  of  the  heels. 

Directions. — The  entire  remaining  portion  of  skin  should  now  be  care- 
fully removed  from  the  limb.  Should  it  be  intended  to  study  from  the 
same  preparation  the  parts  contained  within  the  hoof,  this  must,  before 
the  removal  of  tlie  skin,  be  detached  by  foi'ce  in  the  manner  described 
on  page  35,  The  various  structures  are  now  to  be  defined  by  dissection 
in  the  order  of  the  following  description ;  and  while  the  vessels  and 
nei-ves  are  being  cleaned,  care  must  be  taken  of  the  small  lumhricales 
muscles,  which  lie  on  the  tendon  of  the  deep  flexor  above  the  fetlock. 
The  palmar  arterial  arches  cannot  be  fully  exposed  at  this  stage  of  the 
dissection,  but  it  is  convenient  to  describe  them  here,  from  their 
relationship  to  the  vessels  of  the  region.  The  same  applies  to  the 
large  metacarpal  artery  and  the  plantar  nerves  behind  the  carpus, 
all  of  which  can  be  fully  traced  in  the  examination  of  the  carpal  sheath 
(page  33). 

The  Large  Metacarpal  Artery  (Plate  9).  This  is  the  largest 
artery  in  the  part  of  the  limb  now  exposed,  and  is,  by  means  of  its  ter- 


28  THE  ANATOMY  OF  THE  HORSE. 

minal  branches,  the  main  vessel  of  supply  to  the  digit.  It  has  already 
been  seen  at  its  origin,  as  the  larger  of  the  two  terminal  branches  of  the 
posterior  radial  artery ;  and,  indeed,  from  its  volume  and  direction,  it 
might  be  described  as  the  direct  continuation  of  that  vessel.  From  its 
point  of  origin  at  the  lower  end  of  the  radius,  it  descends  in  company 
with  the  flexor  tendons,  by  passing  behind  the  carpus  and  beneath  the 
carpal  arch.  Emerging  from  beneath  the  last-named  structure,  it  con- 
tinues to  descend  on  the  inner  side  of  the  flexor  tendons  until  a  little 
above  the  fetlock,  where  it  sinks  slightly  inwards  to  bifurcate  into  the 
digital  arteries.  From  the  carpus  downwards  the  artery  is  related  to 
the  internal  metacarpal  vein,  wliich  ascends  in  front  of  it,  and  to  the 
internal  plantar  nerve,  which  is  in  contact  with  it  posteriorly.  The 
relative  position  of  the  three  structures  should  be  carefully  noted  in 
reference  to  the  higher  operation  of  neurotomy.  Only  two  of  its 
collateral  branches  are  of  sufficient  size  to  merit  description,  and  both 
are  somewhat  irregular  in  their  origin.  The  first  of  these  comes 
off"  near  the  origin  of  the  parent  vessel,  and  may  come  from  the 
posterior  radial  itself  It  crosses  behind  the  lower  extremity  of  the 
radius,  and  anastomoses  witli  the  tennination  of  the  ulnar  artery  to 
form  the  supracarpal  or  snperficial  palmar  arch.  The  second  is  an 
un-named  vessel  which  springs  from  the  large  metacarpal  at  or  near  its 
point  of  bifurcation,  and  divides  into  branches  that  ascend  to  anastomose 
with  the  interosseous  metacarpal  arteries. 

The  Supracarpal  or  Superficial  Palmar  Arch  is  formed  behind  the 
lower  extremity  of  the  radius,  by  the  junction  of  the  above-mentioned 
branch  of  the  large  metacarpal  artery  with  the  termination  of  the  ulnar. 
The  convexity  of  the  arch  is  turned  downwards,  and  from  it  there  arise 
several  branches.  The  largest  and  most  regular  of  these  descends 
within  the  carpal  arch,  and  joins  the  small  metacarpal  artery  to  form 
the  subcarpal  or  deep  palmar  arch,  which  will  be  dissected  at  a  later 
stage. 

The  Digital  Arteries  (Plates  9  and  10)  are  the  terminal  branches 
of  the  large  metacarpal  artery.  They  separate  at  an  acute  angle,  the 
outer  one  passing  above  the  fetlock,  between  the  deep  flexor  and  the 
suspensory  ligament.  Each  passes  over  the  side  of  the  fetlock-joint,  and 
descends  at  the  edge  of  the  flexor  tendons  as  far  as  the  inner  face  of 
the  basilar  process,  where  it  bifurcates  to  fonn  the  plantar  and  pre- 
plantar  arteries.  Each  artery  is  related  in  front  to  the  vein  of  the 
same  name,  and  behind  to  the  posterior  branch  of  the  plantar  nerve. 
The  anterior  branch  of  the  same  nerve  crosses  the  vessel  at  the 
fetlock ;  while  other  twigs  cross  over  the  artery  and  form  the 
middle  branch,  which  will  be  found  between  the  artery  and  vein,  or 
resting  on  the  former.  Crossing  these  vessels  and  nerves  obliquely, 
is   a   small    glistening    ligamentous    cord   (Plate    9)    which   stretches 


DISSECTION    OF    THE    ANTERIOR    LIMB.  29 

downwards  and  forwards  from  the  homy  spur  behind  the  fetlock, 
becoming  attached  within  the  wing  of  the  os  pedis.  A  knowledge  of 
these  relationships  is  of  importance  for  the  performance  of  the  lower 
operation  of  neurotomy.  The  collateral  branches  of  the  digital 
arteries  are  : — 

1.  At  different  levels  numerous  small  branches  for  the  skin,  tendons, 
or  articvilations.  Among  these  may  be  included  the  rameaux  echelcmnes 
of  Bouley  (Plate  10).  These  branches,  some  of  them  of  considerable 
size,  spring  from  the  posterior  aspect  of  the  artery,  and  anastomose 
across  the  back  of  the  digit  with  corresponding  branches  from  the  oppo- 
site side,  forming  arches  arranged  like  the  steps  of  a  ladder. 

2.  The  Perx)endicular  Artery^  Avhich  comes  off  at  a  right  angle  about 
the  middle  of  the  first  phalanx,  and  divides  almost  immediately  into  an 
ascending  and  a  descending  set  of  bi-anches,  both  of  which  are  distri- 
buted on  the  front  of  the  first  phalanx.  Branches  from  each  of  these 
sets  anastomose  with  corresponding  vessels  from  the  opposite  side. 

3.  The  Artery  of  the  Plantar  Cushion. 

4.  Vessels  forming  the  Coronary  Circle. 

The  last  two,  as  well  as  the  terminal  branches  of  the  digital  arteries, 
will  be  described  in  connection  with  the  foot. 

The  Small  Metacarpal  Artery  (Plate  9).  This,  the  smaller 
terminal  branch  of  the  posterior  radial  artery,  descends  behind  the 
knee  and  towards  its  inner  side.  It  is  superficially  placed  to  the 
fibrous  band  completing  the  carpal  arch,  while  the  large  metacarpal 
lies  beneath  that  structure.  In  company  with  it  is  the  first  part  of  the 
median  vein.  At  the  level  of  the  head  of  the  inner  metacarpal  bone  it 
crosses  to  the  outer  side  by  passing  between  the  suspensory  ligament 
and  the  subcarpal  ligament,  or  check -band  furnished  from  the 
back  of  the  carpus  to  the  tendon  of  the  deep  flexor.  It  here  anasto- 
moses Avith  a  branch  already  described  as  descending  from  the  supra- 
carpal  arch.     In  this  way  the  subcarpal  arch  is  formed. 

The  Subcarpal  or  Deep  Palmar  Arch  gives  off  the  following  two 
pairs  of  arteries  : — 

1.  The  Anterior  or  Dorsal  Interosseous  Metacarpal  Arteries. — These 
are  small  vessels  (Plate  9),  one  on  each  side  of  the  limb,  which  turn 
forward  round  the  heads  of  the  small  metacarpal  bones,  and  descend  in 
the  grooves  between  these  bones  and  the  large  metacarpal.  They 
supply  the  skin  and  subjacent  structures  on  the  front  of  the  metacarpus, 
and  anastomose  above  the  fetlock  with  divisions  of  the  artery  springing 
from  the  large  metacarpal  at  its  point  of  bifurcation. 

2.  The  Posterior  or  Palmar  Interosseous  Metacarpal  Arteries. — These 
descend  on  the  edge  of  the  suspensory  ligament,  each  being  internally 
placed  to  the  small  metacarpal  bone  of  its  own  side.  They  anastomose 
like  the  preceding,  and  supply  small  branches  to  the  suspensory  ligament 


30  THE  ANATOMY  OF  THE  HORSE. 

and  flexor  tendons.  One  of  them  gives  off  the  mitrient  artery  of  the  hxrge 
metacarpal  bone.     They  are  of  unequal  size,  the  outer  being  the  larger. 

The  Digital  Veins  (Plate  9).  These  are  the  satellites  of  the  digital 
arteries,  in  front  of  which  they  ascend.  They  drain  away  the  blood 
from  the  venous  plexuses  within  the  hoof,  and,  uniting  with  one 
another  above  the  fetlock,  they  foi'm  an  arch  between  the  deep  flexor 
and  the  suspensory  ligament.    From  this  arch  spring  the  metacarpal  veins. 

The  Metacarpal  Veins  are  three  in  number : — 

1.  The  Internal  Metacarpal  Vein  (Plate  9),  which  is  the  largest  of 
the  three,  ascends  in  front  of  the  large  metacarpal  ai'tery,  on  the  inner 
edge  of  the  flexor  tendons.  At  the  inner  side  of  the  back  of  the 
carpus  it  is  continued  as  the  median  vein. 

2.  The  External  Metacarpal  Vein  is  similarly  disposed  on  the  outside 
of  the  flexor  tendons,  in  company  with  the  external  plantar  neI•^•e.  At 
the  carpus  it  divides  into  several  anastomosing  branches,  which  are 
continued  as  the  ulnar  and  posterior  radial  veins. 

3.  The  Interosseotis  or  Deep  Metacarpal  Vein  is  an  irregular  vessel 
ascending  between  the  suspensory  ligament  and  the  inner  splint  bone. 
At  the  back  of  the  carpus  it  breaks  up  into  branches  that  anastomose . 
with  the  external  and  internal  metacarpal  veins. 

The  Plantar  Nerves  {metacarpal  nerves  of  Percivall). — These  are  the 
nerves  which  confer  sensibility  on  the  digit,  and  which,  in  their  main 
trunks,  or  in  one  of  their  terminal  branches,  are  cut  in  the  operation  of 
neurotomy.  They  must  therefore  be  dissected  with  great  care,  and  the 
student  must  make  himself  thoroughly  acquainted  with  their  situation 
and  relations. 

The  Internal  Plantar  Nerve  (Plate  7).  This  is  one  of  the  ter- 
minal branches  of  the  median  nerve.  Beginning  at  a  variable  point 
above  the  carpus,  it  passes  within  the  carpal  arch,  in  close  company  with 
the  large  metacarpal  artery,  both  resting  on  the  side  of  the  deep  flexor 
tendon.  Here  the  nerve  crosses  beneath  the  artery,  to  place  itself 
behind  it.  Throughout  the  metacarpal  region  the  same  relationship 
is  preserved,  the  nerve  lying  immediately  behind  the  artery,  in  front 
of  which  is  the  internal  metacarpal  vein.  Just  above  the  fetlock  the 
artei'y  sinks  in  somewhat  more  deeply  than  the  vein  and  nerve,  and 
thereby  allows  these  to  approach  each  other.  In  the  higher  operation 
of  neurotomy  the  nerve  is  cut  a  little  way  above  the  fetlock,  and  before 
it  divides.  About  the  middle  of  the  metacarpus  it  gives  off*  a  consider- 
able branch  which  winds  obliquely  downwards  and  outwai'ds  behind  the 
flexor  tendons,  to  join  the  external  plantar  nerve  an  inch  or  more  above 
the  button  of  the  splint  bone.  At  the  level  of  the  sesamoid  bones  the 
tnmk  of  the  nerve  divides  into  thi-ee  digital  branches,  which  are 
distinguished  as  anterior,  middle,  and  posterior.  These  are  of  very 
unequal  size,  the  posterior  being  much  the  largest,  and  also  the  most 


DISSECTION    OF    THE    ANTERIOR    LIMB.  31 

important,  as  it  is  the  nei've  which  is  cut  in  the  lower  operation  of 
neurotomy  when  performed  for  navicular  arthritis.  The  middle  is  the 
smallest  and  most  irregular,  and  all  three  branches  are  in  close  relation- 
ship with  the  digital  vessels. 

The  Anterior  branch  descends  in  front  of  the  vein,  distributes  cutane- 
ous branches  to  the  front  of  the  digit,  and  terminates  in  the  coronary 
cushion. 

The  Middle  branch,  Avhich  is  small  and  irregular,  descends  between 
the  artery  and  vein.  It  is  generally,  as  in  Plate  9,  formed  by  the 
union  of  several  smaller  branches  which  cross  forwards  over  the  artery 
before  uniting,  and  it  terminates  in  the  sensitive  laminse  and  coronary 
cushion. 

The  Posterior  branch  lies  close  behind  the  artery,  except  at  the  fetlock, 
where  the  nerve  is  almost  superposed  to  the  artery.  It  accompanies  the 
digital  artery  into  the  hoof,  and  passes  with  the  preplantar  branch  of 
that  vessel  to  be  distributed  to  the  os  pedis  and  the  sensitive  laminfe. 
"Within  the  hoof  it  gives  off  several  branches,  which  for  the  most  part 
accompany  the  arteries. 

The  External  Plantar  Nerve  (Plate  9).  This  is  formed  by  the 
fusion  of  the  termination  of  the  ulnar  nerve  with  one  of  the  terminal 
branches  of  the  median.  These  two  branches  unite  at  the  upper  border 
of  the  pisiform  bone,  beneath  the  middle  flexor  of  the  metacarpus. 
Behind  the  carpus  the  nerve  inclines  downwards  and  outwards,  in  the 
texture  of  the  annular  ligament  that  completes  the  carpal  sheath.  In 
the  metacarpal  region  it  occupies,  on  the  outside  of  the  limb,  a  position 
on  the  flexor  tendons  analagous  to  that  of  the  internal  plantar  nerve 
on  the  inside.  Unlike  the  latter  nerve,  however,  it  is  accompanied  by 
only  a  single  vessel — the  external  metacarpal  vein,  which  lies  in  front  of 
it.  An  inch  or  more  above  the  button  of  the  splint  bone  it  is  joined  by 
the  oblique  branch  from  the  internal  nerve.  In  the  higher  operation 
of  neurotomy  it  is  cut  at  the  same  point  as  the  inner  nerve.  At  the 
level  of  the  sesamoid  bones  it  divides  into  three  digital  branches,  exactly 
similar  to  those  of  the  internal  nerve  already  described. 

The  plantar  nerves  give  filaments  to  the  lumbricales  and  interossei 
muscles,  and  to  the  suspensory  ligament. 

Directions. — The  student  must  now  pursue  the  dissection  of  the 
following  muscles  which  have  already  been  dissected  in  the  fore -arm, 
viz.,  the  extensor  pedis  and  extensor  suffraginis  on  the  front  of  the 
limb,  and  the  superficial  and  deep  flexors  behind.  In  addition  to  these, 
there  are  the  lumbricales  and  interossei  muscles,  which  entirely  belong- 
to  this  region ;  and,  as  they  are  of  small  size,  and  might  easily  be 
overlooked,  their  dissection  must  be  first  undertaken. 

The  Lumbricales  Muscles  (Plate  9)  receive  their  name  in  the 
human  hand  from  their  resemblance  to  a  common  earthworm.     In  the 


32  THE  ANATOMY  OF  THE  HORSE. 

liorse  they  arc  of  small  but  very  vai-iable  size.  Frequently  they  contain 
but  little  muscular  tissue,  but  now  and  again  a  subject  is  met  in  which 
they  are  very  distinct.  They  are  two  in  number,  one  being  placed  on 
each  side  of  the  deep  flexor  tendon,  above  the  fetlock.  The  fibres  of 
the  small  muscular  belly  (uise  from  the  side  of  the  deep  flexoi-,  and 
terminate  in  a  small  tendon  which  is  lost  in  the  tissue  beneatli  the 
horny  spur  of  the  fetlock. 

The  IxTEROSSEi  Muscles  (Plate  9).  These  are  the  representatives 
of  the  muscles  which,  in  the  human  hand,  fill  up  the  interspaces  of  the 
metacarpal  bones,  and  give  lateral  movement  to  the  fingers.  In  the 
hoi'se  they  are  two  in  number,  and  are  extremely  rudimentary'.  Each 
is  to  be  sought  to  the  inner  side  of  the  small  metacarpal  bone  of  its 
own  side,  between  that  bone  and  the  edge  of  the  suspensory  ligament. 
Each  has  at  its  upper  end  a  small  muscular  belly  taking  origin  from 
the  neighbourhood  of  the  head  of  the  small  metacarpal  bone.  It  is 
succeeded  by  a  long,  slender,  nerve-like  tendon,  which  at  the  fetlock 
blends  with  the  band  sent  from  the  suspensory  ligament  to  the  extensor 
l>ediii  tendon,  or  with  the  connective-tissue  on  the  side  of  the  joint, 

The  interossei  and  lumbricales  muscles  are  of  great  interest  to  the 
comparative  anatomist,  but,  from  their  small  size,  they  can  have  no 
appreciable  effect  on  the  movements  of  the  digit. 

The  Tendon  of  the  Extensor  Suffraginis  (Plate  7)  is  to  be  followed 
from  the  point  below  the  carpus  to  which  it  has  already  been  dissected. 

The  flat  tendon,  after  crossing  the  carpus,  descends  to  the  outer  side 
of  the  anterior  surface  of  the  large  metacarpal  bone.  As  it  passes  over 
the  fetlock-joint,  it  becomes  somewhat  broader,  and  its  play  over  the 
anterior  ligament  of  the  joint  is  facilitated  by  means  of  a  small  synovial 
bursa.  Immediately  below  the  joint  it  is  inserted  into  the  fore  part  of 
the  upper  end  of  the  first  phalanx.  In  the  region  of  the  metacarpus 
the  tendon  receives  on  each  side  a  reinforcing  band.  The  outer  band 
comes  from  the  external  side  of  the  carpus ;  the  inner  is  detached  from 
the  extensor  pedis  tendon. 

Action. — The  muscle  is  primarily  an  extensor  of  the  digit  on  the  meta- 
carpus. When  contraction  is  carried  beyond  this,  it  extends  the  meta- 
carpus on  the  fore-arm. 

Tlie  Tendon  of  the  Extensor  Pedis  (Plate  7).  This  tendon,  after 
throwing  off  the  slip  to  the  extensor  suffraginis,  descends  over  the  front 
of  the  metacarpus  and  digit,  and  lies  on  the  middle  line.  Its  play  over 
the  anterior  ligament  of  the  fetlock  is  fiicilitatcd  by  a  small  synovial 
Ijursa ;  while,  over  the  front  of  the  interphalangeal  joints,  the  synovial 
membrane  is  directly  supported  by  the  deep  face  of  the  tendon,  there  being 
no  anterior  ligament  for  these  joints.  At  the  middle  of  the  first  jjhalanx 
the  tendon  is  joined  on  each  side  by  a  strong  band  which  descends 
obliquely  over  tlie  side  of  the   fetlock  from  the  suspensory  ligament. 


DISSECTION    OF    THE   ANTERIOR    LIMB. 


33 


Tlie  tendon   is   finally  inserted  into  the   pyramidal   process   of  the   os 
pedis. 

Action. — The  first  action  of  the  muscle  is  to  extend  the  third 
phalanx  on  the  second,  and  then 
the  second  on  the  first.  When  con- 
traction is  continued,  it  produces 
successively  extension  of  the  fet- 
lock and  of  the  carpus. 

Directions. — The  tendons  on  the 
back  of  the  metacarpus  and  digit  ^ 
must  next  be  dissected ;  and  as 
a  preliminary  step,  the  carpal  , 
and  metacarpo-phalangeal  sheaths 
formed  in  connection  with  these 
tendons  should  be  examined. 

The  Carpal  Sheath  (Fig.  l)is  the 
tubular  passage  through  which  the 
flexors  of  the  digit  are  transmitted 
behind  the  carpus.  It  is  formed 
in  front  by  the  back  of  the  carpus 
covered  by  the  posterior  common 
ligament  of  that  joint.  Behind  it 
is  bounded  in  its  outer  third  by 
the  pisiform  bone,  and  in  its  inner 
two-thirds  by  a  strong  fibrous  band 
representing  the  anterior  annular 
ligament  of  the  human  wrist.  This 
band  stretches  like  an  arch  from 
the  pisiform  bone  to  the  inner  side 
of  the  carpus.  It  is  continuous 
above  with  the  deep  fascia  on  the 
back  of  the  fore-arm,  of  which  it 
may  be  considered  a  thickened 
portion ;  and  below  it  becomes 
thinner,  and  is  continued  as  the 
fascia  on  the  back  of  the  meta- 
carpus (^palmar  fascia  of  man). 
The  carpal  sheath  is  provided  with 

Dissection  of  the  Metacarpus  and  Digit,  showing  the  Tendons  and  their  Synovial 
Sheaths  (Chauveau). 
1.  Synovial  bursa  of  the  extensor  metacarpi  magnus ;  2.  Superior  cul-de-sac,  or  pouch,  of  the 
synovial  membrane  of  the  carpal  sheath  ;  2',  2'.  Inferior  part  of  the  same  ;  3.  Pouch  of  theradio- 
cjirpal  synovial  membrane,  appearing  as  a  hernia  between  the  posterior  common  ligament  and  the 
outermost  radio-carpal  ligament ;  4.  Synovial  bursa  of  the  extensor  pedis  ;  5.  Protrusion  of  the 
synovial  membrane  of  the  fetlock -joint ;  6,  7,  8.  Superior,  middle,  and  inferior  p'ouches  of  the 
synovial  membrane  of  the  metacarpo-phalangeal  sheath  ;  9.  Inferior  extremity  of  the  same,  exposed 
by  the  removal  of  the  reinforcing  sheath  of  the  perforans  tendon  ;  E.  S.  Extensor  suffraginis  ;  S.  L. 
Subcarpal  ligament ;  E.  P.  Extensor  pedis  ;  S.  S.  Superior  sesamoidean  (suspensory)  ligament ;  F. 
Pa.  Flexor  perforans  ;  F.  Pt.  Flexor  perf oratus. 

D 


34  THE  ANATOMY  OF  THE  HORSE. 

a  synovial  membrane,  wliich  lines  it,  and  is  reflected  over  the  flexor 
tendons  to  facilitate  their  gliding.  If  the  fibrous  band  just  described  be 
cut,  and  a  probe  be  passed  upwards  and  downwards  within  the  sheath, 
an  idea  of  the  extent  of  the  synovial  sac  will  be  gained.  It  will  be 
found  to  extend  upwai-ds  for  two  or  three  inches  above  the  carpus, 
and  downwards  as  far  as  the  middle  of  the  metacarpus. 

Directions. — The  fibrous  band  should  be  entirely  removed  in  order  to 
permit  the  examination  of  the  tendons,  and  of  the  nerves  and  bloodvessels 
which  accompany  these  within  the  sheath. 

The  Metacarpo-phalangeal  or  Great  Sesamoid  Sheath  (Fig.  1).  This 
is  a  second  synovial  apparatus  developed  in  connection  with  the  flexor 
tendons.  If  a  vertical  incision  be  made  through  the  superficial  flexor  just 
above  the  fetlock,  and  a  probe  passed  into  the  incision,  it  will  enter  the 
synovial  cavity,  and  may  be  pushed  upwards  for  two  or  three  inches 
above  the  fetlock,  and  downwards  as  far  as  the  middle  of  the  second 
phalanx.  The  synovial  membrane  lubricates  the  pulley-like  surface 
formed  by  the  sesamoid  bones  and  the  inter-sesamoid  ligament,  and  is 
reflected  on  to  the  tendons.  It  is  supported  laterally  by  a  fibrous 
expansion  which,  adhering  to  the  superficial  flexor  behind,  is  inserted 
in  front  by  three  slips  on  each  side,  the  highest  insertion  being  into  the 
sesamoid,  and  the  other  two  into  the  first  phalanx.  At  its  lower  extremity 
this  synovial  membrane  meets  that  of  the  navicular  sheath,  and  in  front 
of  the  same  point  it  is  separated  from  the  synovial  capsiile  of  the  coflin- 
joint  by  a  kind  of  partition  of  yellow  fibrous  tissue  connecting' the  front 
of  the  perforaus  tendon  to  the  back  of  the  os  coronee  (Plate  10,  fig.  2). 

The  Superficial  Flexor  tendon  (Plates  5,  9,  10,  and  11).  The 
tendon  succeeds  the  fleshy  portion  of  the  muscle  at  the  lower  part  of  the 
fore-arm,  and  it  is  there  reinforced  by  a  fibrous  band  which  springs  from 
the  back  of  the  radius  and  is  sometimes  termed  the  superior  car j)al  ligament, 
in  contradistinction  to  the  band  which  reinforces  the  tendon  of  the  deep 
flexor  below  the  carpus.  The  tendon  passes  through  the  carpal  sheath 
in  company  Avith  and  behind  the  deep  flexor,  and  then  descends  behind 
the  metacarpus.  Having  arrived  at  the  fetlock,  there  is  formed  in  it 
a  remarkable  ring,  through  which  the  tendon  of  the  deep  flexor  plays. 
It  is  in  consequence  of  this  arrangement  that  the  superficial  muscle  is 
tei-med  perforates,  and  the  deep  one  perforans.  As  already  seen,  the 
tendons  are  here  enveloped  by  the  synovial  membrane  of  the  meta- 
carpo-phalangeal sheath.  At  its  extremity  the  tendon  is  bifid,  and 
each  slip  is  inserted  into  the  upper  extremity  of  the  second  phalanx  on 
its  lateral  aspect. 

Action. — The  muscle  flexes  successively  the  pastern,  fetlock,  and 
carpal  joints. 

The  Deep  Flexor  tendon  (Plates  5,  9,  10,  and  11)  is,  through- 
out its  course,  closely  related  to  the  preceding,  in  front  of  which  it  lies. 


DISSECTION    OP    THE    ANTERIOR    LIMB.  35 

After  descending  through  the  cai-pal  sheath,  it  is  joined  by  a  very  strong 
fibrous  band — the  inferio7-car2ml  lig ament,  y^'hich.  is  the  downward  continua- 
tion of  the  posterior  common  ligament  of  the  carpus.  This  fuses  with 
the  tendon  about  the  middle  of  the  metacarpus,  and  it  is  of  consider- 
able importance,  being  frequently  involved  in  what  is  commonly  termed 
"  sprain  of  the  back  tendons."  In  that  condition  it  may  be  very  distinctly 
felt  by  manipulating  in  front  of  the  flexor  tendons,  just  below  the  carpus. 
The  tendon,  as  thus  reinforced,  descends  between  the  suspensory  liga- 
ment in  front,  and  the  perforatus  tendon  behind ;  and  at  the  fetlock  it 
glides  over  the  sesamoid  pulley,  and  passes  through  the  ring  of  the 
superficial  flexor.  It  then  passes  between  the  terminal  branches  of  the 
last-mentioned  muscle,  glides  over  the  smooth  surface  on  the  back  of 
the  second  phalanx,  plays  over  the  navicular  bone,  and  finally  becomes 
inserted  into  the  semilunar  crest  of  the  os  pedis.  The  terminal  portion 
of  the  muscle,  as  well  as  the  navicular  sheath  developed  in  connection 
with  it,  will  be  examined  with  the  parts  contained  within  the  hoof. 

Action. — The  muscle  flexes  successively  the  interphalangeal  joints,  the 
fetlock,  and  the  carpus. 

THE    FOOT. 

Directions. — By  the  term  foot,  as  here  applied,  is  meant  the  hoof  and 
the  parts  contained  within  it.  If  it  is  intended  to  study  this  in  a  limb 
the  whole  of  which  is  to  be  successively  dissected,  the  student  must 
proceed  in  the  following  manner.  When  the  dissection  of  the  fore-arm 
has  been  completed,  and  before  the  removal  of  the  skin  from  the  meta- 
carpus and  digit,  the  hoof*  must  be  forcibly  removed  by  the  aid  of  a 
shoeing-smith's  hammer,  toe-knife,  and  pincers.  To  facilitate  this,  the 
hoof  may  be  heated  in  a  fire,  the  skin  of  the  digit  being  swathed  in  a 
wet  cloth  to  prevent  charring.  This  is  the  speediest  method  of  removing 
the  hoof,  but  it  has  the  double  disadvantage  of  destroying  in  great 
measure  the  hoof  itself,  and  also  the  injection  of  the  vessels,  provided 
that  has  been  executed.  The  following  is  a  preferable  method  of  pro- 
cedure : — Procure  a  foot  severed  a  few  inches  above  the  fetlock,  and 
inject  the  arteries  and  veins  from  the  metacarpal  vessels.  When  the 
injection  has  solidified,  roll  the  foot  in  a  piece  of  wet  cloth,  and  bury  it 
in  a  fermenting  heap  of  stable  manure.  Decomposition  will  speedily 
set  in,  and  after  a  week  the  preparation  should  be  examined  at  intervals 
of  two  or  three  days,  the  metacarpal  bone  being  fixed  in  a  vice  while 
forcible  attempts  are  made  to  pull  off  the  hoof.  When  this  has  been 
effected,  the  foot  and  removed  hoof  should  be  immersed  for  a  day  in  a 
saturated  solution  of  carbolic  acid  in  water,  to  which  a  little  methylated 
spirit  may  be  added.  This  will  speedily  remove  all  odour  of  decom- 
position, and  dissection  may  then  be  proceeded  with. 

The  Hoof  (Plate  10,  figs.  4  and  6).  This  is  made  up  of  the  wall,  the 
bars,  the  sole,  and  the  frog. 


3G  THE  ANATOMY  OF  THE  HORSE. 

The  Wall  is  that  i)art  of  the  hoof  which  is  exposed  when  the  foot 
rests  in  its  natural  position  on  a  flat  surface.  It  is  divided,  though 
not  h\  any  well-defined  boundaries,  into  toe,  quarters,  and  heels.  The  toe 
includes  an  area  on  each  side  of  the  middle  line  of  the  wall  in  front ; 
and  it  passes  on  each  side  into  the  quarter,  which  comprises  the  lateral 
region  of  the  wall.  Posteriorly  the  wall  changes  its  direction,  and 
disappears  from  view,  forming  an  angular  part,  which  is  termed  the  heel. 
In  rcalitv,  the  wall  docs  not  stop  at  the  heel,  and  it  is  this  concealed 
continuation  that  is  termed  the  bar.  In  a  well-formed  hoof  the  wall  in 
the  region  of  tlic  toe  slopes  at  an  angle  of  about  50°. 

The  External  Surface  of  the  wall  is,  in  a  state  of  nature,  covered  by  a 
kind  of  epithelial  varnish  termed  the  periople,  which  is  thickest  at  the 
top  of  the  wall,  just  under  the  hair.  This,  Avhich  is  a  natural  varnish 
provided  to  check  evaporation  and  consequent  cracking  of  the  subjacent 
horn,  is  generally  rasped  away  by  the  shoeing-smith.  The  internal  sur- 
face of  the  wall  is  traversed  in  a  vertical  direction  by  the  series  of  hoi-ny 
lamince.  These  number  about  five  or  six  hundred ;  and  before  separa- 
tion of  the  hoof,  they  were  interleaved  with  the  sensitive  laminae  to  be 
presently  described.  The  superior  hoi^ler  of  the  wall  shows  a  kind  of 
gutter,  termed  the  cutigeral  groove,  which  is  the  mould  left  by  the 
coronary  cushion.  The  floor  of  this  groove  has  a  closely  punctated 
appearance,  each  minute  perforation  being  the  xipper  end  of  one  of  the 
horn  tubes  of  the  wall,  and  lodging,  in  the  natural  state,  one  of  the 
papilla)  of  the  coronary  cushion.  The  inferior  border  embraces  the 
sole,  and  in  the  unshod  animal  comes  into  contact  with  the  gi-ound. 

The  wall  is  thicker  at  the  toe  than  at  the  quarters  or  heels ;  and  in 
each  of  these  areas,  it  is  thicker  on  the  outside  than  in  the  correspond- 
ing area  on  the  inside. 

The  Bars.  These  are  the  reflected  terminations  of  the  wall  behind  the 
heels;  and  if  the  foot  be  turned  up,  the  continuity  will  be  distinctly  seen. 

The  Outer  Surface  of  the  bar,  Avhich  is  here  seen,  slojjes  towards  the 
frog,  and  bounds  outwardly  the  lateral  lacuna  of  that  body.  It  shows 
an  inferior  border,  which  runs  towards  the  centre  of  the  sole,  but  stops 
a  little  behind  the  point  of  the  frog.  The  bai-s  are  also  seen  in  the 
interior  of  the  hoof,  where  they  show  an  inta-nal  surface  bearing  horny 
lamime  like  those  of  the  wall.  The  sujyerior  border  of  the  bars  is 
included  between  the  frog  and  the  sole,  and  blended  with  them. 

The  Sole  presents  an  inferior  face,  which  is  vaulted,  and  this  inde- 
pendently of  any  paring  to  which  the  foot  may  have  been  subjected,  as 
the  horn  of  whicli  it  is  composed  exfoliates  so  as  to  give  it  this  con- 
figuration naturally.  The  superior  face  is  somewhat  convex,  and  has 
a  punctated  appearance  similar  to  that  already  seen  in  the  cutigeral 
groove.  The  minute  holes  lodge  the  papillfc  of  the  so-called  sensitive 
sole,  which  is  the  horn  secreting  structure  of  this  region.     Anteriorlij 


DISSECTION    OF    THE    ANTERIOR    LIMB.  37 

the  sole  presents  a  convex  border,  which  unites  it  intimately  to  the 
lower  border  of  the  wall,  a  line  of  whitish  horn  marking  the  junction  of 
the  two  structures.  Posteriorly  it  has  a  deep  V  shaped  indentation, 
into  the  central  point  of  which  the  frog  penetrates,  while  behind  that  on 
each  side  it  is  related  to  the  bar. 

The  sole  of  the  hind  hoof  is  distinguished  from  that  of  the  fore  by 
being  more  vaulted,  and  by  being  more  pointed  (less  circular)  at  the  toe, 
this  latter  difference  affecting  also  the  form  of  the  wall  in  the  same 
region.  The  outer  edge  of  the  sole  is  more  convex  than  the  inner, 
which  enables  one  to  readily  distinguish  between  a  right  and  a  left  hoof. 
The  Frog.  This  is  a  distinctly  elastic  mass  of  horn  which,  in  a  state 
of  nature,  projects  sufficiently  to  come  into  contact  with  the  ground, 
and  thus  give  the  animal  a  secure  foothold.  Its  inferior  surface  shows 
posteriorly  a  shallow  cleft,  or  depression,  termed  the  median  lacunci. 
The  lateral  lacnme  lie  at  the  sides  of  the  frog,  the  outer  boundary  of 
each  lacuna  being  formed  by  the  bar.  The  suj^erior  surface  shows, 
vertically  over  the  median  lacuna,  a  projection  termed  the  frog-stay. 
On  each  side  of  the  frog-stay  this  surface  is  depressed,  and  the  whole  is 
moulded  on  the  plantar  cushion.  This  surface  is  punctated,  and  the 
papillae  of  the  plantar  cushion  are  received  into  the  minute  apertures 
The  posterioi'  extremity,  or  base,  of  the  frog  consists  of  two  rounded  emi- 
nences— the  hulhs,  or  glomes — separated  from  each  other  by  the  median 
lacuna.  The  anterior  extremity,  or  point,  is  wedged  into  the  centre  of  the 
sole.  The  lateral  borders  bring  the  frog  into  relation  with  the  bars  and 
the  sole,  and  there  is  an  intimate  union  with  each  of  these  at  the  point 
of  contact. 

Minute  Structure  of  the  hoof.  The  entire  hoof  is  an  aggregation  of 
modified  epithelial  cells,  which  here  repx'esent  the  horny  layer  of  the 
epidermis.  When  a  thin  section  across  the  wall,  sole,  or  frog  is 
examined,  the  horn  substance  is  seen  to  be  arranged  in  the  form 
of  tubes,  cemented  together  by  an  intertubular  substance,  and 
containing  within  their  lumen  a  quantity  of  intratubular  material. 
All  of  these — tubular,  intertubular,  and  intratubular — are  composed  of 
modified  epithelial  cells,  differing  in  the  three  situations  in  the  direction 
of  the  cells,  their  state  of  aggregation,  or  the  presence  or  absence  of 
contained  pigment.  The  tubes  of  the  wall  are  straight,  and  extend 
parallel  to  the  surface,  from  the  coronary  to  the  inferior  edge  of  the 
wall.  The  tubes  of  the  sole  have  the  same  disposition,  but  those  of  the 
frog  are  slightly  flexuous.  The  upper  end  of  each  tube  is  occupied  by 
an  elongated  vascular  papilla,  Avhich  belongs,  in  the  case  of  the  wall,  to 
the  coronai-y  cushion  ;  in  the  periople,  to  the  perioplic  ring ;  and  in 
the  sole  and  frog,  to  the  sensitive  structures  of  the  same  names.  In  the 
growing  hoof  the  bond  of  connection  between  these  papillated  surfaces 
(which  represent  the  corium  of  the  skin)  and  the  corresponding  part  of 


38  THE  ANATOMY  OF  THE  HORSE. 

the  hoof,  is  a  stratum  of  soft  protoplasmic  epithelial  cells  by  whose 
growth  and  multiplication  the  hoof-horn  is  fomied.  This  stratum  of 
cells  represents  the  deepest  cells  of  the  rete  mucosum  in  the  skin,  and 
it  is  bv  its  ready  decomposition  that  the  bond  of  connection  between 
the  sensitive  and  insensitive  structures  is  destroyed,  permitting  the 
extremity  of  the  digit  to  be  extracted  from  its  horny  investment. 

Directions. — The  student  should  next  turn  his  attention  to  the  ex- 
tremity of  the  digit  as  exposed  by  the  removal  of  the  hoof,  and  he  will 
find  it  to  present  a  configuration  not  unlike  the  exterior  of  the  hoof  itself 
(Plate  10,  figs.  1  and  5).  And  in  the  first  place,  let  him  examine  that 
part  which  he  will  easily  recognise  as  having  been  separated  from  the 
inner  siiiface  of  the  wall.  This  is  traversed  by  a  series  of  leaves  which, 
in  contradistinction  to  those  already  seen  on  the  inner  surface  of  the 
wall,  are  termed  the  sensitive  laminae,  and  sometimes  the  podophyllous. 
tissue. 

The  Sensitive  LAMixiE.  Each  lamina  is  fixed  by  one  of  its  borders  to  ' 
the  periosteum  of  the  os  pedis,  and  extends  in  a  vertical  direction  from 
near  the  coronary  cushion  to  the  sharp  edge  of  the  bone,  whei'e  it 
terminates  in  five  or  six  long  papillae.  In  the  natural  state  the  sensitive 
and  the  horny  lamina)  are  interleaved,  and  the  former  here  represent 
the  corium,  or  true  skin.  The  lamina),  it  will  be  noticed,  become  pro- 
gressively shorter  as  they  are  traced  backwards  ;  and  at  the 'end  of  the 
series  on  each  side,  and  adjacent  to  tlie  plantar  cushion,  there  is  a  number 
of  small  leaves  that  were  interleaved  with  the  horny  lamina)  of  the  bars. 
The  CoROXARY  Cushion.  This  is  a  projecting,  cornice-like  structure, 
placed  above  the  lamina)  and  below  the  limits  of  the  skin  of  the  digit. 
It  fits  into  the  cutigeral  groove  at  the  upper  border  of  the  wall,  and  its 
surface  is  closely  set  with  long  papilla)  which  Avere  received  into  the 
apertures  found  in  that  gi-oove.  These  papillae  give  the  coronary 
cushion  a  velvety  pile,  which  may  be  rendered  very  evident  by  immei-s- 
ing  the  foot  in  water.  If  the  coronary  cushion  be  traced  backwards,  it 
will  be  seen  to  pass  into  the  plantar  cushion.  Above  the  cushion  is  a 
narrow  groove  separating  it  from  the  periopolic  ring.  Below  the  cushion 
there  is  a  narrow  smooth  space  which  runs  between  the  cushion  and  the 
sensitive  lamina).  The  coronary  cushion  is  a  modified  portion  of  the 
corium,  and  through  the  agency  of  the  cells  which  cover  the  surface  of 
its  papilla),  the  wall  of  the  hoof  is  formed. 

The  Perioplic  Ring.  This  ring  is  composed  of  papill.T)  like  those  of 
the  coronary  cushion,  but  smaller  in  size  ;  and  it  is  by  its  agency  that 
the  periople  which  covers  the  exterior  of  the  Avail  is  formed. 

The  Plantar  Cushion.  This  is  a  fibro-elastic  pad  intei'posed  between 
the  horny  frog  and  the  terminal  part  of  the  perforans  tendon.  It 
possesses  two  faces,  two  borders,  a  base,  and  an  apex.  The  lower  face 
looks  backwards  as  well  as  downwards  when  the  foot  rests  on  a  flat 


95  ^ 


PLATE   X. 

Fig.  I. — The  Digit  with  the  Hoof  removed,  flexed  and  viewed  from  behind. 
A.  Sensitive  sole  ;  B.  Sensitive  laniinse  that  were  interleaved  with  the  horny  laminae  of  the  bar  ; 
F.  The  pyramidal  body,  or  sensitive  frog  ;  L.  Lateral  lacuna  of  the  same  ;  M.  Median  lacuna  of  the 
same  ;  Q.  Q.  Fibrous  sheath  uniting  the  two  brandies  of  the  perforatus  ;  R.  Branches  of  the  per- 
foratus  passing  to  be  inserted  into  the  os  coronse  ;  T.  Tendon  of  the  perforatus  ;  T'.  Tendon  of  the 
perforans  in  its  passage  between  the  branches  of  the  perforatus  ;  V.  Reinforcing  sheath  of  the 
plantar  aponeurosis ;  X.  Attachment  of  the  same  to  the  side  of  the  os  suffraginis. 

Fig.  II. — Vertical  me.sial  Section  of  the  Digit. 
A.  Os  pedis;  B.  Coronary  cushion;  C.  Coffin-joint;  D.  Navicular  bone;  E.  Os  coronse; 
F.  Pastern-joint ;  H.  Branch  of  the  perforatus  at  its  insertion  into  the  lateral  aspect  of  the  os 
coronae  ;  I.  Insertion  of  the  plantar  aponeurosis  into  the  semilunar  crest ;  K.  Os  suffraginis ;  L.  The 
perforatus  tendon  ;  M.  Ligament  of  yellow  fibrous  tissue  which  unites  the  anterior  face  of  the 
perforans  to  the  posterior  face  of  the  os  coronas,  and  separates  the  inferior  cul-de-sac  of  the  great 
sesamoid  sheath  from  that  of  the  synovial  membrane  of  the  coffin-joint ;  N.  Protrusion  of  the 
synovial  membrane  of  the  corono-pedal  joint  between  the  navicular  bone  and  the  os  pedis ; 
O.  Small  sesamoid  sheath  ;  P.  Synovial  membrane  of  the  coffin-joint  in  contact  superiorly  with  the 
great  sesamoid  sheath,  from  which  it  is  separated  by  the  yellow  transverse  ligament  M.  ;  T.  Tendon 
of  the  perforans  ;  Y.  Fetlock-joint. 

Fig.  III. — Arteries  of  the  Digit. 
A.  A.  Digital  artery ;  C.  Perpendicular  artery  at  its  origin  ;  H.  One  of  the  posterior  branches 
{i-ameaux  e.chelonncs),  for  the  perforans  tendon  ;  J.  Another  of  the  same  ;  K.  Origin  of  the  artery 
of  the  plantar  cushion  ;  M.  Origin  of  anterior  branch  of  coronary  cime ;  Ji.'  Posterior  branch  of  the 
same  circle  ;  R.  Origin  of  preplantar  artery  ;  S.  Plantar  art^Ky  in  the  plantar  groove  and  in  the  os 
pedis,  forming  with  the  opposite  artery  the  semilunar  anastomosis  ;  V.  V.  Descending  branches  from 
the  semilunar  anastomosis. 


Fig.  IV.— The  Hoof— plantar  aspect. 
P.  P.  Region  of  the  toe  ;  S.  Sole  ;  L.  Frog ;  A.  Line  indicating  the  junction  of  wall  and  sole  ; 
B.  Angle  of  inflexion  of  the  wall,  showing  the  continuity  of  the  wall  and  the  bar  ;  E.  Inferior  edge 
of  the  bar  ;  F.  Lateral  lacuna  of  tlie  frog ;  G.  Bulbs  of  the  frog ;  Q.  Median  lacuna  of  the  frog ; 
U.  Regions  of  the  quarters  ;  O.  Regions  of  the  heels. 

Fig.  v.— Extremity  of  the  Digit  with  the  Hoof  removed — viewed  from  the  side. 
A.  B.  Plantar  cushion  with  its  villosities  ;    D.   Groove  between  the  plantar  ciishion  and  the 
perioplic  ring  ;  E.  Perioplic  ring  ;  F.  Inferior  border  of  the  plantar  cushion  ;  G.  Sensitive  laminaj, 
or  podophyllous  tissue  ;  H.  Villosities  which  terminate  the  laminre. 

Fig.  VI-— Antero-posterior  mesial  Section  of  the  Hoof— showing  its  interior. 
M.  Series  of  horny  laminae  ;  O.  Section  of  the  wall ;  P.  Section  of  the  sole  ;  S.  Upper  edge  of  the 
periople  above  the  cutigeral  groove  ;  T.  Section  of  the  frog  ;  X.  Cutigeral  groove. 


40  DISSECTION    OF    THE    ANTERIOK    LIMJ5. 

surface,  and  it  is  moulded  on  the  upper  face  of  the  horny  frog,  to  which 
it  has  a  close  resemblance  in  form.  The  central  portion  of  the  cushion 
is  therefore  sometimes  termed  the  sensitive  frog,  and  it  is  also  known  as 
the  pyramidal  body.  It  shows  in  front  a  single  ridge,  which  posteriorly 
becomes  divided  into  two  by  a  deep  median  cleft  for  the  reception  of 
the  frog-stay.  This  surface  has  a  villous  aspect,  the  papillfe  being 
imbedded  in  the  foramina  seen  on  the  upper  surfiice  of  the  horny  frog. 
The  horny  frog  is  formed  by  the  agency  of  the  cells  covering  these 
papilla).  The  upptr  face  looks  forwards  as  well  as  upwards,  and  is 
applied  to  the  reinforcing  sheath  of  the  deep  flexor  tendon.  The 
borders,  w^hich  are  right  and  left,  bring  the  plantar  cushion  into  relation 
with  the  inner  surface  of  the  lateral  cartilages.  The  apex  lies  in  front 
of  the  semilunar  crest  of  the  os  pedis,  with  whose  periosteum  the  tissue 
of  the  cushion  is  intimately  blended.  The  base  of  the  cushion  consists 
of  two  thick  ronnded  masses  tei-med  the  bulbs  of  the  plantar  cushion. 
These  are  continuous  in  front  with  the  ridges  of  the  pyramidal  body, 
and  they  present  the  same  velvety  aspect ;  while,  on  each  side,  the 
villous  tissue  joins  the  extremities  of  the  coronary  cushion. 

The  Sensitive  Sole.  The  student  shoidd  next  examine  that  part  of 
the  foot  which,  before  separation  of  the  hoof,  came  into  contact  with 
the  upper  surface  of  the  horny  sole,  and  which  for  that  reason  is  termed 
the  sensitive  sole.  It  is  of  a  roughly  crescentic  form,  being  penetrated 
by  the  pyramidal  body  behind ;  and  it  is  co-extensive  with  the  plantar 
surface  of  the  os  pedis.  Its  connective-tissue  basis  is  firmly  adherent  to 
the  periosteum  of  the  bone,  Avhile  its  free  surface  bears  long  papilhe 
which  penetrate  the  horn  tubes  of  the  sole.  The  hoi-ny  sole  is  formed 
by  the  agency  of  the  cells  which  clothe  the  papilla;  of  the  sensitive  sole. 

Directions. — On  manipulating  the  bulbs  of  the  plantar  cushion,  the 
student  will  feel  the  lateral  cartilages  of  the  foot ;  and  one  of  these  is 
to  be  exposed  and  defined  by  removing  one  half  of  the  plantar  cushion. 

The  Lateral  Cartilages.  These  ai"e  in  the  main  composed  of 
hyaline  cartilage,  though  often  erroneously  tei-med  the  fibro-cartilages  of 
the  foot.  As  is  common  with  fibro-cartilage  in  many  other  regions,  it 
shows  a  transitional  structure  at  its  periphery,  where  its  matrix  becomes 
more  or  less  fibrous.  Each  plate  of  cartilage  possesses  two  faces,  and 
four  borders  separated  by  four  angles.  The  exterrud  face  is  convex  and 
covered  by  a  plexus  of  veins,  some  of  which  penetrate  the  plate  and 
connect  the  plexus  with  another  lying  beneath  it.  The  internal  face  is 
concave.  Behind  it  is  united  to  the  plantar  cushion,  while  anteriorly 
it  protects  the  corono-pedal  articulation ;  and  a  cul-de-sac  of  the  synovial 
membrane  of  the  joint  lies  in  direct  contact  with  the  cartilage,  a  fact 
which  it  is  important  to  remember  in  connection  with  operations  for 
"quittor."  The  suj^erior  border  is  thin  and  flexible,  and  may  be  felt  in 
the  living  animal.     The  digital  vessels  cross  this  border  in  passing  into 


THE  ANATOMY  OF  THE  HORSE. 


41 


the  foot.  The  inferior  border  is  supported  by  the  wing  of  the  os  pedis  in 
front,  while  posteriorly  it  blends  with  the  plantar  cushion.  The  anterior 
border  slopes  downwards  and  backw^ards,  and  is  blended  with  the  antero- 
lateral ligament  of  the  corono-pedal  joint.  The  posterior  border  is  parallel 
to  the  anterior,  and  is  covered  by  the  plantar  cushion.  The  four  borders 
meet  at  four  angles,  of  which  the  postero-superior  one  and  the  one  diago- 
nally opposite  are  obtuse,  while  the  other  two  are  acute. 

In  the  disease  termed  "  Side-bones,"  the  lateral  cartilages  lose  their 
mobility,  in  consequence  of  their  conversion  into  bone. 

The  Bloodvessels  of  the  Foot  (Plate  10,  fig.  3).  These  should  be  studied 
in  an  injected  limb  from  which  the  hoof  has  been  removed  by  the  method 
of  decomposition  described  at  page  35.  The  arteries  of  the  foot  are 
derived  from  the  digital  artery,  which  has  already  been  dissected  in  its 
descent  towards  the  foot,  where,  within  the  wing  of  the  os  pedis,  it 
divides  into  the  plantar  and  preplantar  arteries.  Some  of  the  collateral 
branches  of  the  digital  artery  have  already  been  described  at  page  29 ;  but 
there  remain  for  examination  the  artery  of  the  plantar  cushion  and  the 
coronary  circle,  as  well  as  the  plantar  and  preplantar  terminal  branches. 

The  Artery  of  the  Plantar  Cushion  arises  from  the  digital,  just  as 
that  vessel  passes  within  the  upper  border  of  the  lateral  cartilage,  and 
it  passes  obliquely  downwards  and  backwards  to  its  destination.  Besides 
supplying  the  plantar  cushion,  it  gives  off  a  branch  which  turns  forwards 
to  concur  in  the  formation  of  the  circumflex  artery  of  the  coronary 
cushion. 

The  Coronary  Circle.  Where  each  digital  artery  lies  under  cover 
of  the  lateral  cartilage,  it  gives  off  an  anterior  and  a  posterior  branch 
which  inosculate  on  the  middle  line  before  and  behind  with  the  corre- 
sponding branches  of  the  opposite  side,  and  thus  form  an  arterial  circle. 
This  circle  closely  embraces  the  os  coronse ;  and  among  the  largest 
branches  furnished  by  it,  are  two  which  emanate  from  its  anterior  half, 
and  descend,  one  at  each  border  of  the  extensor  tendon,  to  aid  in  form- 
ing the  circumflex  artery  of  the  coronary  cushion. 

The  Circumflex  Artery  of  the  Coronary  Cushion  (Chcmveau).  This  is 
a  slender  vascular  arch  placed  immediately  above  the  coronary  cushion,  to 
which  its  branches  are  distributed.  It  is  fed  in  front  by  the  two  above- 
mentioned  vessels  from  the  coronary  circle,  and  behind,  on  each  side,  by 
the  before-mentioned  branch  from  the  artery  of  the  plantar  cushion. 

The  Preplantar  Artery  is  the  smaller  of  the  two  terminal  branches 
of  the  digital.  It  passes  forwards  through  the  notch  in  the  wing  of  the 
OS  pedis,  and  then  along  the  preplantar  groove  on  the  laminal  surface  of 
that  bone,  where  its  branches  are  expended  in  the  sensitive  laminae. 

The  Plantar  Artery  passes  along  the  plantar  groove  to  enter  the 
foramen  of  the  same  name.  Within  the  os  pedis  it  inoscidates  with  the 
corresponding  vessel  of  the  opposite  side,  forming  the  plantar  arch,  or 


■11'  DISSECTION    OF   THE    ANTERIOR    LIMB. 

semilunar  (tnastomosis.  From  this  intra-osseoiis  arcli  a  great  number  of 
branches  proceed.  An  ascending  (anterior  laminal)  set  of  these  leave 
the  OS  pedis  by  the  numerous  small  foramina  which  cribble  its  laminal 
surface.  A  descending  (inferior  comimniicating)  set  escape  from  the 
bone  by  the  series  of  larger  foramina  which  open  on  the  sharp  edge 
separating  its  laminal  and  plantar  surftices.  These  inferior  communi- 
cating arteries  anastomose  right  and  left  with  each  other^  and  thus  form 
the  circumflex  artery  of  the  toe.  From  the  concavity  of  this  artery 
branches  pass  backwards,  and  supply  the  tissue  of  the  sole. 

The  Veins  of  the  Foot. — Intra-osaeous  vessels.  Within  the  os  pedis 
the  arterial  branches  are  accompanied  by  satellite  veins.  There  is 
thus  a  semilunar  venous  anastomosis,  to  which  small  veins  converge 
from '  the  laminal  surface  of  the  bone.  The  blood  from  this  sinus  is 
drained  away  by  a  larger  vessel  which  passes  out  by  the  plantar  fora- 
men in  company  with  the  plantar  artery,  and  joins  the  posterior  part  of 
the  coronary  plexus.  Extra-osseons  vessels.  The  foot  is  richly  provided 
with  a  superficial  system  of  vessels,  which  are  arranged  in  the  foi'm  of  a 
close-meshed  network  iiaving  little  or  no  conuuunication  with  the  deep 
set.  This  venous  envelope  of  the  foot  is  divided  into  a  solar,  a  lami?ial 
(podophyllous),  and  a  coronary  plexus.  Where  the  solar  and  laminal 
plexuses  meet,  a  composite  venous  vessel  runs  in  company  with  the 
circumflex  artery  of  the  toe.  These  two  plexuses  communicate  freely 
with  each  other,  and  with  the  coronary  plexus.  This  last  consists  of  a 
central  pai't,  wliich  underlies  the  coronary  cushion,  and  of  two  lateral 
parts,  which  on  each  side  ramify  on  both  surfaces  of  the  lateral  cartilage. 
By  the  convergence  of  branches  belonging  to  this  cartilaginous  division 
of  the  coronary  plexus,  the  digital  veins  are  formed  ;  and  these  drain 
away  the  blood  from  both  the  intra-osseous  and  extra-osseous  systems 
of  vessels. 

Directions. — The  terminal  portion  of  the  deep  flexor  tendon,  and  the 
synovial  apparatus  developed  in  connection  with  it,  should  now  be 
examined. 

The  Deep  Flexor  tendon  {Plates  10  and  11),  when  it  reaches  the 
upper  border  of  the  navicular  bono,  widens  out  to  form  what  is  called  the 
jilantar  aponeurosis.  This  plantar  aponeurosis  plays  over  the  navicular 
bone  by  means  of  the  navicular  sheath,  and  is  covered  posteriorly  by  a 
fibrous  layer  which  ultimately  blends  with  it.  It  becomes  inserted  into 
the  semilunar  crest  of  the  os  pedis,  and  into  the  bone  behind  that  crest. 
The  above-mentioned  fibrous  layer  was  first  described  by  Bouley,  and 
designated  by  liim  the  reinforciwi  sheath  of  the  perforans.  This  expansion 
is  attached  on  each  side  by  a  slip  to  the  lower  half  of  the  first  phalanx, 
and  it  serves  to  maintain  the  pl^itar  aponeurosis  against  the  navicular 
bone. 

The  Navicular  or  Small  Sesamoid  Sheath  (Plate  10,  fig.  2).    This  is  a 


THE  ANATOMY  OF  THE  HORSE.  43 

synovial  apparatus  developed  in  connection  with  the  perforans  tendon 
where  it  plays  over  the  navicular  bone.  It  lines  the  deep  face  of  the 
tendon,  and  is  reflected  on  to  the  navicular  bone  and  interosseous  liga- 
ment. It  also  extends  above  the  navicular  bone,  where  it  is  in  contact 
with  the  synovial  membrane  of  the  coffin -joint  and  that  of  the  metacarpo- 
phalangeal sheath. 

THE    SHOULDER-JOINT. 

This  joint  is  formed  between  the  glenoid  fossa  of  the  scapula  and  the 
head  of  the  humerus.  It  is  enclosed  by  a  single  capsular  ligament 
lined  internally  by  the  synovial  membrane.  The  absence  of  lateral  or 
other  retaining  ligaments  in  connection  with  the  joint,  is  compensated 
for  by  the  numerous  tendons  which  pass  from  one  bone  to  the  other  in 
close  relation  to  the  capsular  ligament.  These  muscles  are  as  follows  : — 
the  siipraspinatus,  infraspinatus,  teres  minor,  biceps,  and  small  scapulo- 
humeralis.  The  last  passes  over  the  joint  behind,  where  some  of  its 
fibres  seem  to  be  inserted  into  the  ligament.  In  front  of  the  joint  the 
tendon  of  the  biceps  is  separated  from  the  ligament  by  a  pad  of  fat. 

Movements. — The  joint  belongs  to  the  class  of  enarthrodial  or  ball- 
and-socket  joints,  and  the  amount  of  its  mobility  should  be  proved  by 
manipulation  before  the  removal  of  the  muscles.  If  the  scapula  be  kept 
fixed,  it  will  be  found  that  the  humerus  can  be  carried  backwards  so  as 
to  diminish  the  angle  formed  by  the  meeting  of  the  bones.  This  is  a 
movement  of  flexion.  Or  the  humerus  can  be  carried  forward  in  the 
same  plane  as  in  the  preceding  movement,  but  increasing  the  angle. 
This  is  extension.  Or  ■  again,  the  humerus  may  be  moved  in  a  lateral 
direction  either  outwards  or  inwards.  When,  in  the  living  animal,  it 
is  caiTied  inwards,  the  limb  is  thrown  towards  the  middle  plane  of  the 
body,  and  is  said  to  be  adducted.  The  opposite  movement,  by  which 
the  limb  is  carried  outwards  from  the  middle  plane,  is  termed  abduction. 
Two  other  movements  are  permitted  in  the  joint,  viz.,  rotation  and 
circumduction.  In  rotation  the  humerus,  without  change  of  place  as  a 
whole,  turns  round  its  own  axis.  In  circumduction  the  shaft  of  the 
humerus  moves  so  as  to  describe  the  surface  of  a  cone. 

(These  diff"erent  terms  having  been  here  defined  at  length,  their 
application  in  the  case  of  the  other  joints  of  the  body  will  be  readily 
understood). 

The  shoulder-joint  of  the  horse  is  thus  possessed  of  considerable 
freedom  of  movement ;  but  still,  the  range  of  its  mobility,  owing  to 
the  absence  of  ,a  clavicle,  and  to  the  different  disposition  of  the  pectoral 
muscles,  is  much  more  restricted  than  in  the  human  arm. 

Directions. — The  muscles  which  surround  the  joint  must  now  be 
removed,  care  being  taken  not  to  cut  the  capsular  ligament. 

The  Capsular  Ligament  loosely  surrounds  the  articular  ends  of  the 


44  DISSECTION    OF    THE    ANTERIOR    LIMB. 

bones,  and  may  be  conceived  as  having  the  form  of  a  double-mouthed 
sack,  one  mouth  being  attached  around  the  rim  of  the  glenoid  cavity, 
and  the  other  at  the  periphery  of  the  head  of  the  humerus.  The  wall 
of  this  sack  is  comparatively  thin,  but  in  front  it  is  strengthened  by 
accessoi-y  fibres  that  pass  in  a  divergent  manner  from  the  coracoid  pro- 
cess to  the  outer  and  inner  tuberosities.  These  correspond  to  the 
coraco-humeral  ligament  of  man. 

Directions. — If,  in  the  removal  of  the  muscles,  the  ligament  has  been 
preserved  perfectly  intact,  it  will  be  noticed  that  though  a  considerable 
force  be  exerted  to  pull  the  articulai-  surfaces  from  each  other,  they  still 
remain  in  contact.  If,  however,  an  incision  be  made  in  the  ligament,  the 
air  will  be  heard  to  rush  into  the  joint,  while  the  bones  separate  to  the 
extent  of  half  an  inch  or  more.  In  the  shoulder  then,  as  in  other  joints, 
atmospheric  pressure  is  to  be  included  among  the  agents  keeping  the 
articular  surfaces  in  contact.  The  capsular  ligament  is  to  be  slit  up  so 
as  to  expose  the  smooth  and  glistening  aspect  of  the  synovial  membrane, 
and  the  articular  surfiices  of  the  bones  covered  by  articular  cartilage. 

The  Synovial  Membrane  lines  the  inner  surface  of  the  capsular 
ligament.  It  secretes  the  synovia,  or  joint  oil,  some  of  which  will  be 
seen  escaping  from  the  joint. 

THE    ELBOW-JOINT  (PLATE  11,  fig.   1). 

This  joint  is  formed  by  the  lower  extremity  of  the  humerus  and  the 
upper  extremities  of  the  bones  of  the  fore-arm.  It  possesses  two  lateral 
ligaments,  and  an  anterior  ligament  which  supports  the  synovial 
membrane  in  front ;  but  behind,  there  being  no  ligament,  the  synovial 
sac  is  directly  supported  by  muscles. 

Movements. — This  is  a  ginglymoid  joint,  the  only  movements  being 
Jiexion  and  extension.  In  Jiexion,  while  the  humerus  remains  fixed,  the 
bones  of  the  fore-arm  are  cai-ried  forwards  until  the  movement  is  arrested 
by  the  coi-onoid  process  passing  into  the  fossa  of  the  same  name.  In 
this  movement  the  bones  of  the  fore-arm  do  not  move  in  the  plane  in 
which  the  humerus  lies,  but  deviate  a  little  outwards.  The  opposite 
movement  is  exten-non,  in  which  the  radius  and  ulna  are  carried  back- 
wards until  they  are  arrested  by  the  tension  of  the  lateral  ligaments, 
and  by  the  passage  of  the  beak  of  the  olecranon  into  the  fossa  of  the 
same  name. 

Dirediom. — The  anterior  and  lateral  ligaments  are  to  be  exposed 
and  defined  by  removing  the  muscles  from  the  front  of  the  joint,  but  on 
the  posterior  aspect  of  the  joint  the  muscles  should  not  be  removed  at 
present. 

The  External  Lateral  Ligament  is  a  cord-like  band  which  is  fixed 
superiorly  to-  a  depression  on  the  outer  side  of  the  lower  extremity  of 
the  humerus,  and  to  the  ridge  which  forms  the  lower  boundary  of  the 


THE  ANATOMY  OF  THE  HORSE.  45 

musculo-spiral  groove  ;  while  inferiorly  it  passes  to  be  inserted  into  the 
external  tuberosity  at  the  upper  end  of  the  radius. 

The  Internal  Lateral  Ligament  forms  a  longer  but  more  slender 
cord  than  the  preceding,  and  passes  from  a  small  eminence  on  the 
outer  side  of  the  lower  extremity  of  the  humerus  to  be  inserted  into  the 
shaft  of  the  radius  below  the  bicipital  tuberosity.  Some  of  the  anterior 
fibres  join  the  tendon  of  the  bicejjs  or  the  anterior  ligament,  while  some 
of  the  posterior  join  the  arciform  fibres  connecting  the  radius  and  ulna. 

The  Anterior  Ligament  is  of  a  membranous  form.  Its  upper  border 
is  fixed  to  the  humerus,  its  lower  border  to  the  radius,  while  its  lateral 
borders  blend  with  the  lateral  ligaments. 

Directions. — The  anterior  and  lateral  ligaments  should  now  be  cut 
transversely  about  their  middle  in  order  to  expose  the  interior  of  the  joint. 

The  Synovial  Membrane  will  be  seen  to  line  the  inner  face  of  the 
anterior  and  lateral  ligaments,  but  at  the  back  part  of  the  joint  there 
is  no  ligament  and  the  membrane  is  suppoi-ted  by  the  muscles.  If  the 
finger  be  passed  backwards  and  upwards,  it  will  enter  a  process  of  the 
synovial  capsule  which  extends  upwards  into  the  olecranon  fossa,  Avhere 
a  pad  of  fat  intervenes  between  it  and  the  anconeus  muscle.  Just 
behind  the  external  lateral  ligament  the  membrane  lines  the  origin  of 
the  flexor  metacarpi  externus.  On  the  inner  side  of  the  joint,  behind 
the  internal  lateral  ligament,  the  membrane  lines  the  tendons  of  origin 
of  the  middle  and  internal  flexors  of  the  metacarpus,  and  of  the  super- 
ficial and  deep  flexors  of  the  digit.  This  disposition  of  the  synovial 
capsule  will  be  rendered  more  evident  by  cutting  the  above-mentioned 
muscles  a  few  inches  below  the  joint,  and  then  turning  their  tendons  of 
origin  upwards. 

Directions. — The  humerus  being  now  completely  severed  from  the 
radius  and  ulna,  the  mode  of  union  of  these  latter  bones  should  be 
examined. 

The  Radio-ulnar  Articulation. — In  the  adult  animal  the  bones  of 
the  fore-arm  are  fused  together  below  the  radio-ulnar  arch,  by  ossification 
of  the  interosseous  fibres  which  in  the  young  animal  are  interposed 
between  the  two  bones.  Above  the  arch,  however,  the  fibres  interposed 
between  the  bones  do  not  ossify  except  in  a  very  old  animal,  but  persist 
as  an  interosseous  ligament.  The  union  of  the  two  bones  is  further 
maintained  by  arciform  fibres  passing  on  each  side  from  the  one  bone  to 
the  other,  and  blending  with  the  lateral  ligaments  of  the  elbow.  At  the 
upper  part  of  their  opposed  surfaces,  the  two  bones  respond  to  each 
other  by  two  small  synovial  facets,  which,  however,  have  no  special 
synovial  membrane,  but  are  lubricated  by  processes  from  the  synovial 
capsule  of  the  elbows-joint. 

Movements. — These  are  inappreciable,  the  limb  of  the  horse  being 
fixed  in  a  condition  of  pronation. 


4G  DISSECTION*    OF    THE    ANTERIOR    LIMH. 

THE    KNEE,    OR    CARPUS  (PLATE  11,  figS.   2    aud  3). 

This  is  not  a  simple,  but  a  composite,  joint,  and  entering  into  its 
formation  there  are  the  carpal  bones,  the  lower  extremity  of  the  radius, 
and  the  upper  extremities  of  the  bones  of  the  metacarpus.  The  carpal 
bones  are  arranged  in  two  rows,  or  tiers,  and  the  bones  of  each  row  ai'e 
firmly  bound  together  and  converted  into  a  single  piece  by  ligaments 
])assing  between  the  adjacent  bones.  A  transverse  joint  is  then  formed 
between  the  upper  and  the  lower  tier.  This  may  be  called  the  inter- 
carpal joint,  and  it  is  secured  by  special  ligaments  passing  between  the 
two  rows.  Another  transver.se  joint  is  formed  between  the  lower  row 
and  the  heads  of  the  metacarpal  bones ;  and  this,  which  has  also  got 
special  ligaments,  is  termed  the  carpo-metncarpal  articulation.  A  third 
transverse  joint  is  formed  between  the  lower  end  of  the  radius  and  the 
upper  row.  This,  which  is  the  7Y(dio-carpal  joint,  is  also  provided  with 
special  ligaments.  Lastly,  there  are  four  ligaments  which  do  not  belong 
specially  to  any  of  these  articulations,  but  secure  the  stability  of  the 
entire  composite  joint,  and  are  therefore  termed  common. 

Movements. — The  movements  which  take  place  at  the  carpus  are 
flexion  and  extension,  and  each  of  the  transverse  joints  above-mentioned 
is  a  ginglymus.  When  these  movements  are  executed,  however,  the 
thi-ee  joints  do  not  participate  in  them  in  an  equal  degree.  The  largest 
share  of  the  movement  occurs  at  the  radio-carpal  articulation,  and  the 
smallest  between  the  carpus  and  the  metacarpus  ;  while,  as  regards  the 
amount  of  movement,  the  inter-carpal  transverse  joint  occupies  an  inter- 
mediate position.  When  the  limb  is  flexed  at  the  carpus,  it  will  be 
noticed  that  the  metacarpus  and  digit  deviate  a  little  outwards  from  the 
plane  of  the  fore-arm.  When  the  limb  is  fully  extended  the  lateral 
ligaments  are  tightly  stretched,  and  resist  any  attempts  to  produce 
abduction  or  adduction  ;  but  these  movements  can  be  produced  when  the 
limb  is  iwWy  flexed,  in  which  position  the  lateral  ligaments  are  relaxed. 
Lateral  movement,  however,  is  not  executed  at  this  joint  in  any  appreci- 
able degree  in  the  living  animal.  The  gliding  movement  permitted 
between  adjacent  bones  in  each  row  is  of  importance,  as  tending  to 
distribute  pressure,  and  obviate  the  bad  eftects  which  would  have  been 
likely  to  result  from  concussion  had  eacli  row  been  a  single  rigid  mass. 

Directions. — The  tendons  which  pass  in  relation  to  the  joint  before 
and  behind  should  be  removed,  and  the  ligaments  should  be  studied 
in  the  order  of  the  following  description. 

There  are  four  ligaments  common  to  the  whole  joint,  viz.,  two  lateral, 
an  anterior,  and  a  posterior. 

The  External  Lateral  Ligament  is  a  cord-like  band  composed  of  a 
deep  and  a  superficial  set  of  fibres,  which  slightly  cross  each  other.  It 
is  fixed  superiorly  to  the  external  tuberosity  at  the  lower  end  of  the 


PLATE  XI 


Fig.  VI 


Fig.  VII 


i;  Prinied    i.y  ^V  i-A.K  JqWimWii 


J(JINTS  AND  LIG.\MENTS  OF  FORE  LIMB 


PLATE  XI. 

Fig.  I.— Ligaments  of  the  Elbow,  seen  from  behind  (I.ei/h). 
A.  Ext.  lateral  ligament ;   B.  Int.  lateral  ligament ;   C.  0.  C.  Arciform  ligaments  ;   D.  Radio- 


Fig.    II.^LlOAMENTS   OF   THE   CAHPUS,    FRONT  VIEW  (Cliauceau). 

1.   1.   Ant.   ligaments  of  upper  row  ;   2.   An  ant.   ligament  of  lower   row ;    3.  3.  Ant.   carpo- 
metacarpal ligaments  ;  4.  Int.  lateral  ligament ;  5.  Ext.  lateral  ligament. 

Fig.  III.— Ligaments  of  the  Carpus,  viewed  from  the  outer  side  (Chaiireau). 
1.  1.1.  Ant.  ligaments  of  upper  row;  2.  An  ant.  ligament  of  the  lower  row;    3.  3.  Ant.  carpo- 
metacarpal ligaments  ;    4.  An  intercarpal  ligament ;    5.  Ext.  lateral  ligament ;  0.  A  radio-carpal 
ligament. 


Fig.  IV.— Ligaments  of  the  Fetlock,  Pastern,  and  Coffin-Joints  ;  side  view  (Cliauveau). 

1.  Superficial  fasciculus  of  the  ext.  lateral  ligament  of  the  fetlock  ;  2.  3.  Sesamoid  and  phalangeal 
slips  of  the  deep  fasciculus  of  the  same  ligament;  4.  5.0.  Upper,  middle,  and  lower  fibrous  slips 
attaching  the  glenoidal  fibro-cartilage  to  the os suffraginis  ;  7.  Lateral  ligament  of  the  pastern-joint; 
S.  Antero-lateral  ligament  of  the  coffin-joint ;  9.  Postero-lateral  ligament  of  the  same  joint. 

Fig.  V. — Back  of  the  Digit  dissected  to  show  the  Tendons  and  Ligaments  (^Boide)j). 
A.  Antero-lateral  ligament  of  the  coffin-joint ;  B.  Insertion  of  extensor  pedis  tendon  ;  D.  Postero- 
lateral ligament  of  the  coffin-joint ;  E.  Divergent  fibres  of  the  same  ligament  passing  to  be  attached 
to  tlie  wing  of  the  os  pedis  and  inner  surface  of  the  lateral  cartilage  ;  F.  Slip  sent  from  suspensory 
ligament  to  extensor  tendon  ;  P.  Branch  of  bifurcation  of  the  suspensory  ligament ;  R.  Branch  of 
perforatus  ;  T.  Perforans  emerging  from  between  the  branches  of  the  perforatus  ;  Y.  Attachment  of 
tlie  reinforcing  sheath  of  the  perforans  tendon  to  the  side  of  the  os  suffraginis. 

Fig.  VI. — Back  of  the  Digit  dissected  to  show  the  Tendons  and  Ligaments  {Boidcy). 
A.  Superficial  inferior  sesamoidean  ligament ;    B.   Highest  slip   attaching  the  glenoidal  fibro- 
cartilage  of  the  pastern-joint  to  the  first  phalanax  ;  O.  Branch  of  perforatus  ;  P.  Middle  inferior 
sesamoidean  ligament ;  S.  Insertion  of  plantar  aponeurosis  into  semilunar  crest ;  T.  Reinforcing 
sheath  of  the  plantar  aponeurosis  ;  X.  Perforans  tendon. 


Fig.  VII.— Back  of  the  Fetlock-joint  (Modified  from  Boaley). 
A.  Intersesamoid  ligament ;    B.  B.   Lateral  bands  of  the  middle  inferior  sesamoidean  ligament ; 
C.  Middle  band  of  the  same  ligament,  its  upper  attachment  cut  away  to  show  D.  the  deep  inferior 
sesamoidean  ligament. 


48  THE  ANATOMY  OF  THE  HOUSE. 

radius ;  and  passing  over  the  outside  of  the  carpus,  it  furnishes  slips  to 
the  cuneiform  and  unciform  bones,  and  termiiiates  on  the  head  of  the 
external  small  metacarpal  bone.  The  ligament  is  perforated  by  a  thecal 
canal  in  which  the  tendon  of  the  extensor  sufFraginis  plays. 

The  Internal  Lateual  Ligament  is  fixed  superiorly  to  the  internal 
tuberosity  of  the  radius,  and  inferiorly  to  the  heads  of  the  large  and 
inner  small  metacarpal  bones,  furnishing  slips,  as  it  passes  over  the 
carpus,  to  the  scaphoid,  magnum,  and  trapezoid  bones. 

The  Anteuior  Common  Ligament  has  a  flattened,  four-sided  form.  It 
is  fixed  superiorly  to  the  radius,  and  inferiorly  to  the  large  metacarpal 
bone,  while  its  lateral  borders  are  united  to  the  lateral  ligaments.  Its 
deep  fiice  is  partly  adherent  to  the  carpal  bones  or  their  anterior 
ligaments,  and  partly  it  is  lined  by  synovial  membrane.  The  tendons 
of  the  extensor  pedis  and  the  extensors  of  the  metacai-pus  play  over  its 
superficial  face,  where  they  are  provided  with  synovial  bursee.  The 
ligament  is  somewhat  loose  when  the  joint  is  extended,  and  is  put  on 
the  stretch  dui'ing  flexion. 

The  Posterior  Common  Ligament  is  a  much  stronger  ligament  than 
the  preceding.  It  is  fixed  above  to  the  radius,  and  below  to  the  large 
metacarpal  bone.  Its  internal  border  mixes  its  fibres  with  the  internal 
lateral  ligament,  while  its  outer  border  is  blended  in  the  same  way  with 
the  most  external  of  the  intercarpal  ligaments.  Its  anterior  or  deep 
face  is  very  intimately  united  to  the  carpal  bones,  and  its  posterior  fiice 
is  smooth  and  lined  by  the  synovial  membrane  of  the  carpal  sheath. 
The  suhcarjxd  ligament,  or  fibrous  band  wliich  reinforces  the  perforans 
tendon  below  the  carj^us,  takes  origin  from  the  posterior  common  liga- 
ment, or  may  be  described  as  the  downward  continuation  of  that 
ligament. 

Directions. — The  anterior  and  lateral  ligaments  just  described  are  to 
be  carefully  dissected  away,  and  in  removing  the  first  of  these,  care  is 
to  be  taken  of  the  anterior  bands  connecting  the  bones  in  each  row. 

Radio-carpal  Ligaments. — There  are  three  of  these.  The  strongest 
of  them  is  a  thick  cord  that  stretches  obliquely  downwards  and  inwards 
behind  the  carpus,  and  connects  the  radius  and  scaphoid.  It  will  be 
seen,  without  removing  the  posterior  common  ligament,  which  covers  it, 
by  strongly  flexing  the  joint  and  looking  into  it  from  the  front.  The 
second  is  a  very  slender  ligament  which  is  fixed  to  the  radius  beneath 
the  preceding,  and  passes  downwards  to  be  attached  to  the  pisiform  and 
the  interosseous  ligament  luiiting  the  cuneiform  and  semilunar  bones. 
The  third  is  situated  at  the  outside  of  the  carpus,  where  it  connects 
the  radius  and  the  upper  border  of  the  pisifoi*m  bone,  and  is  partly 
covered  by  the  lateral  ligament. 

The  Inter-carpal  Ligaments  are  also  three  in  number.  Two  of 
them  are  situated  behind  the  joint,  under  cover  of  the  posterior  common 


DISSECTIOX    OF    THE    ANTERIOR    LIMB.  49 

ligament,  and  will  be  seen  without  further  dissection  on  flexing  the 
joint  and  looking  into  it  from  the  front.  One  of  these  connects  the 
scaphoid  to  the  magnum  and  trapezoid,  the  other  joins  the  cuneiform 
and  magnum.  The  third  is  a  strong  ligament  situated  at  the  outer 
side  of  the  joint,  where  it  is  blended  with  the  lateral  ligament  in  front, 
and  with  the  posterior  common  ligament  behind.  Its  fibres  are  fixed 
superiorly  to  the  pisiform  bone,  and  inferiorly  to  the  unciform  and  head 
of  the  external  small  metacarpal  bone. 

The  Carpo-metacarpal  Ligaments  are  four  in  number — two  anterior 
and  two  interosseous.  One  of  the  anterior  ligaments  is  composed  of 
two  separate  slips  which  connect  the  os  magnum  and  large  metacarpal 
bone.  The  other  passes  from  the  unciform  to  the  head  of  the  external 
small  metacarpal  bone,  under  cover  of  the  lateral  ligament.  The  two 
interosseous  pass,  one  on  each  side,  from  the  point  of  articulation  of  the 
large  and  small  metacarpal  bones,  to  join  the  interosseous  ligaments 
connecting  the  bones  of  the  lower  row. 

Directions. — Attention  may  at  this  stage  be  given  to  the  disposition 
of  the  synovial  membranes  of  the  carpus,  which  are  three  in  number. 

Synovial  Membranes. — 1.  The  radio-carpal  synovial  membrane  not 
only  facilitates  the  movements  between  the  radius  and  the  bones  of  the 
upper  row,  but  also  descends  between  the  latter  bones  as  far  as  their 
interosseous  ligaments.  2.  The  inter-carpal  synovial  membrane,  in  the 
same  way,  belongs  to  the  intercarpal  transverse  joint ;  but  it  is  also 
insinuated  above,  between  the  bones  of  the  upper  row  as  far  as  their 
interosseous  ligaments,  and  descends  in  the  same  way  below,  between 
the  adjacent  bones  of  the  lower  row.  It  communicates  with  the  next. 
3.  The  carp)0-metacarpal  synovial  membrane  facilitates  the  movements 
between  the  lower  row  and  the  heads  of  the  metacarpal  bones,  ascends 
between  the  adjacent  bones  of  the  lower  row  as  far  as  their  interosseous 
ligaments,  and  dips  down  to  supply  the  articulations  between  the  large 
and  small  metacarpals. 

Directions. — The  radio-carpal,  inter-carpal,  and  posterior  common 
ligaments  should  now  be  cut  transversely.  The  upper  row  will  thus  be 
isolated  as  a  single  piece  for  the  examination  of  its  special  ligaments. 

The  Ligaments  of  the  Upper  Row  are  three  anterior,  and  three 
interosseous ;  and  they  are  extremely  simple.  The  anterior  ligaments 
are  flattened  bands  connecting  the  adjacent  bones  in  front,  while  the 
interosseous  bands  are  very  short  and  connect  the  contiguous  surfaces 
of  the  bones. 

The  Ligaments  of  the  Lower  Row  are  two  anterior,  and  two  inter- 
osseous; and  they  are  disposed  like  those  of  the  upper  row.  In 
examining  these,  the  lower  tier  of  bones  must  not  be  separated  from 
the  metacarpus,  as  that  would  involve  the  destruction,  in  part,  of  the 
suspensory  ligament  of  the  fetlock. 

E 


50  THE  ANATOMY  OF  THK  HORSE. 

The  Inter-metacarpal  Articulations.  The  head  of  the  large  meta- 
carpal bone  responds  to  one  of  the  small  metacarpals  on  each  side  by  a 
small  synovial  joint  lubricated  by  a  process  from  the  carpo-metacarpal 
synovial  membrane.  Below  that  point  the  union  of  the  bones  is  main- 
tained by  short  interosseotis  fibres,  which,  in  adult  animals,  are  very 
frequently  ossified.  The  lower  extremities  of  the  splint  bones,  however, 
for  a  short  distance  above  the  little  knob  that  terminates  them,  remain 
freely  movable,  as  may  be  felt  by  manipulation  in  the  living  animal. 
In  addition  to  the  interosseous  fibres,  the  ligaments  of  the  carpus  which 
get  inserted  in  common  into  the  heads  of  both  large  and  small  metacarpal 
bones,  contribute  to  the  union  of  these  bones. 

the  fetlock-joint  (plate  11,  FIGS.  4-7). 

This,  which  is  technically  termed  the  metacarpo-pUalangeal  articula- 
tion, is  a  ginglymoid  joint ;  and  its  articular  surfaces  are  furnished  by 
the  lower  extremity  of  the  large  metacarpal  bone,  the  upper  extremity 
of  the  first  phalanx,  and  the  two  sesamoid  bones.  It  coiresponds  to  the 
joint  at  the  knuckles  in  the  human  hand. 

Movements. — Flexion  and  extension  are,  in  the  natural  state,  the  only 
movements  executed  at  the  joint;  but  by  manij)ulation,  slight  lateral 
movements  may  be  produced  when  the  joint  is  fully  flexed.  In  com- 
plete extension  the  digit  is  carried  beyond  the  point  at  which  it  lies  in  a 
straight  line  with  the  metacai-pus  (over-extension),  until  the, movement  is 
arrested  by  tension  of  the  suspensory  ligament. 

Directions. — The  tendons  which  pass  in  relation  to  the  joint  before 
and  behind  having  been  carefully  removed,  the  ligaments  should  be 
dissected  and  studied  in  the  order  of  their  description. 

The  Superior  Sesamoidean  or  Suspensory  Ligament. — The  main  por- 
tion of  this  ligament  is  lodged  in  the  channel  formed  by  the  three  meta- 
carpal bones,  where  it  is  related  by  its  posterior  face  to  the  perforans 
tendon  and  its  reinforcing  band  (subcarpal  ligament).  It  has  a  double 
origin  behind  the  carpus,  viz.,  (1)  by  a  superficial  layer  fi'om  the  lower 
row  of  carpal  bones,  and  (2)  by  a  deeper  layer  from  the  upper  end  of 
the  large  metacarpal  bone.  (In  the  hind  limb  it  has  a  similar  origin  from 
the  tarsus  and  metatarsus).  These  two  portions  blend,  and  descend  be- 
hind the  metacarpus  as  a  flattened  band  which  bifurcates  a  few  inches 
above  the  sesamoid  bones.  Each  branch  passes  to  the  sesamoid  bone  of 
its  own  side,  where  a  considerable  proportion  of  its  fibres  become  inserted ; 
while  the  rest  is  continued  in  the  form  of  a  band  which  crosses  obliquely 
downwards  and  forwards  over  the  side  of  tlie  fetlock  to  join  the  extensor 
tendon  on  the  front  of  the  digit,  and  be  continued  with  it  to  the  os  pedis. 
The  ligament  is  composed  of  white  fibrous  tissue  with  a  constant  admix- 
ture of  striped  muscular  tissue.  The  presence  of  muscular  tissue  here, 
points  to  the  conclusion  (strengthened  by  other  considerations)  that  the 


DISSECTION    OF    THE    ANTERIOR    LIMB.  51 

suspensory  ligament  is  a  muscle  which,  in  the  evolution  of  the  horse, 
has  undergone  retrogi-essive  changes,  and  lost  its  original  function.* 

The  Inferior  Sesamoidean  Ligaments.  These  are  three  in  number, 
and  may  be  distinguished  as  superficial,  middle,  and  deep.  The  super- 
ficial ligament  is  fixed  below  to  the  glenoidal  fibro-cartilage  developed 
behind  the  superior  articular  surface  of  the  second  phalanx.  It  ascends 
as  a  flattened  band  behind  the  os  suffraginis,  where  it  is  placed  between 
the  middle  ligament  and  the  tendon  of  the  deep  flexor ;  and,  widening  a 
little,  it  is  inserted  into  the  base  of  the  sesamoids  and  the  intersesamoid 
ligament.  By  cutting  the  ligament  about  its  middle,  and  i-eflecting  it 
upwards  and  downwards,  the  middle  ligament  will  be  brought  into  view. 
The  middle  ligament  consists  of  a  median  and  two  lateral  bands.  Each 
is  fixed  to  the  back  of  the  os  suffi-aginis,  and  ascends  to  be  inserted  into 
the  base  of  the  sesamoids.  This  should  be  cut  and  reflected  like  the 
preceding  ligament,  in  order  to  expose  the  next.  The  deep  ligament 
consists  of  a  few  short  fibres  disposed  like  the  letter  X,  and  fixed,  on  the 
one  hand,  to  the  upper  part  of  the  posterior  surface  of  the  os  suffraginis, 
and,  on  the  other,  into  the  base  of  the  sesamoid  bones.  This  ligament 
supports  the  synovial  membrane  of  the  joint. 

The  Lateral  Ligaments  of  the  fetlock-joint.  Each  comprises  (1)  a 
superficial  fasciculus  connecting  the  lower  extremity  of  the  large 
metacarpal  bone  to  the  upper  extremity  of  the  first  phalanx ;  and  (2)  a 
deep  fasciculus  attached,  on  the  one  hand,  to  the  large  metacarpal 
beneath  the  preceding,  and,  on  the  other,  to  the  sesamoid  and  upper 
extremity  of  the  first  phalanx. 

The  Anterior  Ligament  has  a  membranous,  four-sided  form.  It 
covers  the  joint  in  front,  and  supports  the  synovial  membrane  by  its 
deep  face ;  while  the  extensor  pedis  tendon  passes  over  its  superficial 
aspect,  a  synovial  bursa  being  interposed.  It  is  fixed  above  to  the 
large  metacarpal,  below  to  the  first  phalanx,  and  on  each  side  to  the 
lateral  ligament. 

Directions.- — On  one  side  of  the  joint  the  lateral  ligament  and  the 
slip  sent  from  the  suspensory  ligament  to  the  extensor  tendon  must  be 
removed  to  expose  the  next  ligament. 

The  Lateral  Sesamoidean  Ligaments.  These  are  not  to  be  con- 
founded with  the  lateral  ligaments  of  the  joint,  by  which  they  are 
partly  covered.  Each  fixes  the  sesamoid  of  its  own  side  to  the  upper 
extremity  of  the  first  phalanx. 

The  Intersesamoid  Ligambmt  is  the  name  given  to  the  fibro-cartila- 
ginous  tissue  which  unites  the  two  sesamoids,  and  with  them  forms 
a  pulley-like  surface  for  the  passage  of  the  deep  flexor  tendon. 

The   Synovial   Membrane   is   supported   in   front    by   the   anterior 

*  According  to  Professor  D.  J.  Cunningham  (RrporU  of  the  Challenger  Expedition,  Vol.  V.),  the 
ligament  is  the  altered  flexor  brevis  of  the  middle  digit,  the  corresponding  muscle  in  the  human 
subject  being  tlie  1st  plantar  interosseous  muscle. 


■iZ  THE    ANATOMY    OF    THE    HOKSE. 

ligament,  and  on  each  side  by  the  hiteral  ligament.  Behind  the  joint 
it  is  supported  below  the  sesamoids  by  the  deep  inferior  sesamoidoan 
ligament,  but  above  these  bones  it  is  unsupported ;  and  when  the 
synovial  sac  is  distended,  it  bulges  iip wards  between  the  branches  of 
the  suspensory  ligament  (Fig.  1,  page  33). 

THE  PA.STERN-JOINT  (PLATE   11). 

This  joint,  which  is  technically  termed  the  ^tirst  interphalangeat 
articulation,  is  formed  between  the  distal  end  of  the  os  suffraginis  and 
the  proximal  end  of  the  os  corontc.  It  is  a  ginglymus,  or  hinge  joint, 
and  con-esponds  to  the  second  joint  of  the  human  finger. 

Movements. — As  with  the  joint  last  described,  the  only  natural 
movements  are  Jlej.ion  and  extension. 

Directions. — The  tendon  of  the  extensor  pedis,  which  passes  over 
the  front  of  the  joint,  should  be  cut  and  reflected  downwards.  This 
will  show  that  the  tendon  completes  the  joint  in  front,  where  it  plays 
the  part  of  an  anterior  ligament,  and  sujiports  the  synovial  membrane. 
The  lateral  ligaments  are  next  to  be  defined,  and  after  these,  the 
supplementary  cartilaginous  apparatus  placed  behind  the  joint. 

The  Lateral  Ligaments.  Each  of  these  stretches  from  the  lower 
extremity  of  the  first  phalanx  on  its  lateral  aspect,  to  be  inserted  into 
the  side  of  the  os  coronte,  and  beyond  that  point  some  of  its  fibres  are 
continued  downwards  and  backwards  as  the  postero-latcral  ligament  of 
the  second  interphalangeal  joint. 

The  Glenoidal  Fibro-Cartilage.  This  is  a  piece  of  fibro-cartilage 
fixed  at  the  posterior  edge  of  the  upper  articular  surface  of  the  os 
coronae.  It  serves  to  increase  that  surfoce,  and  its  anterior  ftice  is 
moulded  on  the  lower  articular  surface  of  the  first  phalanx,  while  its 
posterior  face  is  smooth  for  the  passage  of  the  perforans  tendon.  Three 
fibrous  slips  j^ass  from  it  on  each  side,  and  are  attached  to  the  first 
phalanx.  The  superficial  inferior  sesamoidean  ligament  is  inserted  into 
it,  and  the  terminal  insertion  of  the  perforatus  tendon  is  blended  with 
it  on  each  side. 

Synovial  Membrane.  This  is  supported  in  front  by  the  extensor 
tendon,  and  on  each  side  by  the  lateral  ligament.  Posteriorly  it  lines 
the  glenoidal  fibro-cartilage,  and  is  prolonged  upwards  as  a  pouch  behind 
the  lower  extremity  of  the  first  phalanx  (Plate  10,  fig.  2). 

the  coffin-joint  (plate  11). 

This,  the  second  interphalangeal  joint,  has  three  bones  entering  into  its 
formation,  viz.,  the  os  coronre,  the  os  i)cdis,  and  the  navicular  bone.  It 
is  a  ginglymus,  and  corresponds  to  the  fii-st  joint  of  the  human  finger. 

Movements.— /V^.r?c»?i  and  extension. 


DISSECTION    OF    THE    ANTERIOR    LIMB. 


53 


It  possesses  an  interosseous  ligament,  and  two  pairs  of  lateral  ligaments. 

The  Interosseous  Ligament  is  composed  of  shoi-t  fibres  passing  fi-om 
the  inferior  border  of  the  navicular  bone  to  the  os  pedis  behind  its  artic- 
ular surface. 

The  Antero-Lateral  Ligaments.  Each  of  these  passes  from  the  side 
of  the  OS  coronse  to  be  inserted  into  the  excavation  at  the  side  of  the 
pyramidal  process  of  the  os  pedis. 

The  Postero-Lateral  Ligaments.  These  seem  to  be  the  downward  con- 
tinuations of  the  latei-al  ligaments  of  the  pastern-joint.  Passing  from  the 
side  of  the  os  coronse,  each  is  inserted  into  the  upper  border  of  the  nav- 
icular bone,  and  sends  slips  to  the  wing  of  the  os  pedis  and  inner  surface 
of  the  lateral  cartilage. 

Synovial  Membrane.  This  is  supported  in  front  by  the  extensor 
tendon,  and  laterally  by  the  lateral  ligaments.  A  protrusion  of  it  passes  on 
each  side  between  the  antero-lateral  and  postero-lateral  ligaments,  and 
lies  in  relation  to  the  deep  face  of  the  lateral  cartilage.  A  third  protru- 
sion passes  upwards  posteriorly,  between  the  navicular  bone  and  the  back 
of  the  OS  coronse  (Plate  10,  fig.  2). 


TABULAR  VIEW  OF  THE  MUSCLES  OF  THE  FORE  LIMB  IN  THEIR 
ACTION  ON  THE  DIFFERENT  JOINTS. 


/Deltoid. 

I  Coraco-humeralis. 
F  Latissimus  dorsi. 
Flexors       <  Teres  major. 
I  Teres  minor. 

I  Scapulo-humeralis  gracilis  (?) 
\Large  head  of  triceps. 

T,   ,  (  Suprasijinatus. 

Extensors   ■[  Mastoido-humeralis. 

I  Deltoid. 
Abductors  s  Teres  minor. 
( Infraspinatus. 


(  Superficial  pectoral. 
Adductors  -N  Subscapularis. 

i  Coraco-humeralis. 


Rotators 
outwards 


Rotators 
inwards 


•  Deltoid 
Teres  minor 
,  Infraspinatus 


]\Iastoido-humeralis. 
Latissimus  dorsi. 
Teres  major. 


Flexors 


Flexors 


Flexors 


(  Flexor  brachi. 

1  Brachialis  anticus. 


Elbow. 

I  (  Triceps  extensor  cubiti. 

Extensors  <  Anconeus. 

I  ( Scapulo-ulnaris. 


Carpus. 


/Flexor  metacarpi  extermis. 
I  Flexor  metacarpi  medius. 
-  Flexor  metacarpi  internus. 
I  Flexor  perforans. 
^  Flexor  perforatus. 

j  Flexor  perforans. 
^  Flexor  perforatus. 


(  Flexor  perforans. 
\  Flexor  perforatus, 


Extensors 

Fetlock. 

Extensors 
Pastern. 

Extensor - 


/'  Extensor  metacarpi  magniis. 
J  Extensor  metacarpi  obliquus. 
I  Extensor  pedis. 
V  Extensor  suffraginis. 


(  Extensor  pedis. 

(  Extensor  suffraginis. 


Extensor  pedis. 

COFFIX-JOINT. 

Flexor — Flexor  perforans.  |  Extensor— Extensor  pedis. 


54 


THE    ANATOMY    OF    THE    HORSE. 


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DISSECTION    OF    THE    ANTERIOR    LIMB. 


55 


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CHAPTER   11. 

DISSECTION  OF  THE  POSTERIOR  LIMB. 

In  the  male  subject,  the  dissection  of  the  pei-inteum  must  be  completed 
before  the  dissector  of  the  hind  limb  can  begin  his  operations. 

THE    INNER    ASPECT    OP    THE    THIGH. 

Position. — The  animal  should  be  placed  on  the  middle  line  of  its  back, 
and  its  hind  limbs  should  be  drawn  forcibly  upwards  and  outwards  by 
ropes  running  over  pulleys  fixed  to  the  ceiling.  This  is  the  position 
most  convenient  for  allowing  the  dissection  of  both  hind  limbs  to  be 
pursued  at  the  same  time.  If  only  one  limb  is  being  dissected,  the  rope 
may  be  unfostened  from  that  limb,  and  the  body  allowed  to  incline  to 
the  same  side,  as  in  Plate  12. 

Surface-marking. — The  internal  saphena  vein  ascends  on  the  inner 
aspect  of  the  thigh ;  and  a  few  inches  below  the  upper  limit  of  the  region, 
it  dips  in  between  the  sartorius  and  gracilis  muscles.  Pressure  at  this 
point  in  the  living  animal  will  produce  distension  of  the  vessel,  and 
render  its  course  much  more  evident.  Venesection  is  sometimes  per- 
foi-med  on  this  vessel.  Above  the  point  where  the  before-mentioned 
vessel  disappeai-s  from  view,  the  deep  inguinal  lymphatic  glands  are 
situated  in  the  intei-stice  between  the  sartorius  and  gracilis  muscles. 
They  here  cover  the  femoral  artery,  and  may  be  very  distinctly  felt  in  a 
case  oi  lymphangitis,  or  "weed." 

Directions. — An  incision  through  the  skin  is  to  be  carried  down  the 
middle  line  of  the  thigh,  and  terminated  a  few  inches  below  the  level  of 
the  stifle-joint.  Here  another  incision  is  to  be  made  across  the  inner 
aspect  of  the  limb,  from  its  anterior  to  its  posterior  border.  These 
incisions,  together  with  those  already  made  in  the  dissection  of  the 
perinseum,  will  enable  the  dissector  to  reflect  the  skin  as  an  anterior  and 
a  posterior  flap.  The  student  should  then  dissect  the  internal  saphena 
vein  with  its  accompanying  artery  and  nerve,  and  the  cutaneous  nerves 
at  the  forepart  of  the  region,  which  are  derived  from  the  lumbar  nerves. 
Thex-eafter  the  surface  of  the  sartorius  and  gracilis  is  to  be  cleaned,  and 
these  muscles  are  to  be  examined. 

The   Internal   Saphena  Vein    (Plate  12).      This  is   a  large  vessel 


DISSECTION    OF    THE    POSTERIOR    LIMB.  57 

formed  on  the  inner  side  of  the  leg  by  the  junction  of  an  anterior  and  a 
posterior  root,  these  being  the  upward  continuations  of  the  inner  and 
outer  metatarsal  veins.  In  the  thigh  it  inclines  upwards  and  forwards 
on  the  surface  of  the  gracilis,  until  it  disappears  between  that  muscle 
and  the  sartorius,  to  empty  itself  into  the  femoral  vein. 

The  Saphena  Artery  (Plate  12).  This  artery  lies  in  front  of  the 
vein.  It  is  a  long  and  slender  vessel  given  off  by  the  femoral  artery 
about  the  middle  of  the  femur.  It  comes  out  between  the  sartorius  and 
gracilis,  or  it  may  pierce  the  edge  of  one  of  these  muscles.  It  then 
descends  in  front  of  the  saphena  vein,  and  finally  divides  into  two 
branches,  which  accompany  the  roots  of  that  vessel. 

The  Internal  Saphenous  Nerve  (Plate  12)  is  a  branch  of  the  anterior 
crui-al,  from  which  it  is  given  off  a  little  above  the  brim  of  the  pelvis. 
At  the  crural  arch  (Poupart's  ligament)  it  descends  in  front  of  the 
femoral  artery,  to  which  and  the  sartorius  muscle  it  supplies  branches. 
It  then  divides  into  two  cutaneous  branches,  which  emerge  from  between 
the  sartorius  and  the  gracilis,  in  company  with  the  saphenous  artery 
and  vein.  The  anterior  half  of  the  nerve  gives  oflF  branches  for  the 
supply  of  the  thigh  in  front  of  the  vein,  and  is  continued  downwards 
over  the  forepart  of  the  inner  side  of  the  leg,  as  far  as  the  hock. 
The  posterior  half  sends  branches  backwards  for  the  supply  of  the  pos- 
terior part  of  the  thigh,  and  it  then  descends  behind  the  anterior  half. 

Cutaneous  Branches  from  the  lumbar  nerves.  These  will  be  found 
at  the  forepart  of  the  thigh,  the  largest  (from  the  3rd  lumbar)  being 
accompanied  by  the  posterior  division  of  the  circumflex  iliac  artery. 

The  Precrural  Lymphatic  Glands.  These  are  supei-ficially  placed 
at  the  inner  side  of  the  front  of  the  thigh,  on  the  track  of  the  above- 
mentioned  branch  of  the  circumflex  iliac  artery. 

Fascia.  At  the  forepart  of  the  region  now  exposed,  the  muscles  are 
overspread  by  a  strong  membranous  fascia,  which  is  attached  superiorly 
to  the  tendon  of  the  external  oblique  muscle  of  the  abdomen,  at  the  line 
where  it  is  reflected  to  form  Pouparfs  ligament.  Round  the  anterior 
border  of  the  thigh  this  ftiscia  is  continuous  with  the  strong  fascia  lata  ; 
but  when  traced  backwards,  it  becomes  less  fibrous,  and  over  the  posterior 
part  of  the  region  it  is  thin  and  areolar.  When  it  has  been  examined, 
the  fascia  is  to  be  cleaned  away  from  the  subjacent  muscles. 

The  Deep  Inguinal  Lymphatic  Glands  (Plate  45)  are  ten  or  twelve 
in  number,  and  foi-m  a  chain  connected  by  areolar  tissue,  and  situated 
in  the  upper  part  of  the  interstice  between  the  gracilis  and  sartorivis 
muscles,  and  over  the  femoral  vessels. 

The  Sartorius  (Plate  12).  This  is  a  somewhat  slender  muscle  which 
at  present  can  be  dissected  only  in  a  part  of  its  course.  It  is  seen 
descending  beneath  Poupart's  ligament,  from  its  point  of  origin  within 
the  abdominal  cavity.    It  there  takes  origin  from  the  iliac  fascia  (Plate  45). 


58  THE  ANATOMY  OF  THE  HORSE. 

In  the  thigh  it  lies  in  front  of  the  gracilis.  About  the  middle  of  their 
line  of  apposition  the  saphena  vessels  and  nerves  emerge,  but  below  that 
point  the  muscles  are  adherent  to  each  other.  It  is  inserted  into  the 
internal  straight  ligament  of  the  patella. 

Action. — To  adduct  and  flex  the  hip-joint.  To  a  slight  extent  it  may 
also  rotate  the  limb  inwards  at  the  stifle. 

The  Gracilis  (Plate  12).  This  muscle  does  not  possess  the  slender 
character  from  which  it  is  named  in  human  anatomy.  It  is  a  large, 
somewhat  four-sided  mass,  forming  the  greater  part  of  what  is  termed 
the  Jlat  of  the  thigh.  A  linear  depression  seen  on  the  surface  of  the 
muscle  when  it  is  cleaned,  is  often  mistaken  by  students  for  the  line  of 
separation  between  it  and  the  sai-torius.  It  arises  from  the  lower  face 
of  the  pubis  and  ischium  close  to  the  symphysis,  and  it  is  here  united 
to  its  fellow  of  the  opposite  side.  Inferiorly  it  has  a  broad  flat  tendon, 
united  in  front  to  that  of  the  sartorius.  It  is  inserted  with  the  sartorius 
into  the  internal  straight  ligament  of  the  patella,  and  into  a  line  on  the 
tibia  between  its  anterior  and  internal  tuberosities.  The  posterior  edge 
of  its  tendon  is  continuous  with  the  deep  ftiscia  of  the  leg.  A  large 
branch  from  the  external  pudic  veins  traverses  the  muscle  near  its 
origin,  and  opens  into  the  femoral  vein. 

Action. — To  adduct  the  hip,  and  rotate  the  limb  inwards. 

Directions. — The  two  preceding  muscles  are  to  be  carefully  cut  across 
about  their  middle,  and  turned  upwards  and  downwards.  On  reflecting 
the  proximal  half  of  the  gracilis,  branches  of  the  obtui-ator  nerve  and 
deep  femoral  artery  will  be  seen  penetrating  its  deep  face ;  and,  in  the 
same  way,  twigs  from  the  saphena  nerve  will  be  found  entering  the 
sartorius.  The  deep  inguinal  glands  are  to  be  removed,  and  the 
femoral  vessels  and  anterior  crural  nei've  are  to  be  dissected. 

The  Femoral  Artery  (Plate  13)  is  the  main  arterial  trunk  for  the 
supply  of  the  hind  limb.  It  is  the  direct  continuation  of  the  external 
iliac,  the  brim  of  the  pelvis  being  selected  as  the  arbitrary  line  of 
division  between  the  two  vessels  ;  and,  in  like  manner,  it  is  directly  con- 
tinued by  the  popliteal  artery,  the  vessel  changing  its  name  when  it 
passes  between  the  two  heads  of  the  gastrocnemius  muscle.  The  lower 
third  of  the  vessel,  however,  will  not  be  exposed  till  the  next  stage  of 
the  dissection.  The  part  of  the  vessel  now  seen  begins  at  the  pelvic 
brim,  where  it  is  seen  issuing  from  beneath  Poupart's  ligament.  It 
there  rests  on  the  common  termination  of  the  iliacus  and  psoas  magnus, 
having  the  sartorius  in  front  and  the  pectineus  behind.  In  the  thigh  it 
descends  obliquely  downwards  and  backwards,  resting  first  on  the  com- 
mon termination  of  the  iliacus  and  psoas  magnus,  and  then  on  the 
vastus  internus.  It  has  the  sartorius  in  front ;  while  posteriorly  it  is 
related  first  to  the  pectineus,  and  then  to  the  adductor  parvus.  In  this 
course  it  corresponds  to  the  interstice  between  the  gracilis  and  sartorius 


DISSECTION    OF    THE    POSTERIOR    LIMB.  59 

muscles,  and  is  covered  by  the  deep  inguinal  lymphatic  glands.  It  is 
closely  related  to  the  femoral  vein,  which  lies  beneath  and  slightly  pos- 
terior to  it,  except  at  the  brim  of  the  pelvis,  where  the  vein  is  imme- 
diately posterior  to  the  artery.  In  the  present  stage  of  the  dissection 
the  vessel  disappears  between  the  upper  and  lower  insertions  of  the 
adductor  magnus,  where  it  will  subsequently  be  followed.  The  following 
collateral  branches  of  the  femoral  are  here  seen  : — 

1.  The  Profunda  or  Deep  Femoral  Artery.  This  branch  is  given 
off  luider  Poupart's  ligament  at  the  pelvic  brim.  At  its  origin  it 
usually  forms  a  short  common  trunk  with  the  prepubic  artery.  It 
passes  downwards  and  backwards  under  the  pectineus,  and  will  be 
followed  in  the  next  stage  of  the  dissection. 

2.  Muscular  Branches.  The  largest  of  these  is  a  vessel  of  consider- 
able size  for  the  supply  of  the  quadriceps  extensor  cruris  muscle.  It 
comes  off  at  about  the  same  level  as  the  profunda,  which  it  generally 
exceeds  in  volume;  and  passing  over  the  psoas  magnus  and  iliacus,  and 
under  the  sartorius,  it  penetrates  between  the  rectus  femoris  and  vastus 
internus,  in  company  with  the  anterior  crural  nerve.  Other  innominate 
arteries  of  smaller  size  enter  the  vastus  internus,  pectineus,  gracilis, 
sartorius,  and  adductors. 

3.  The  Saphena  Artery  already  described  (page  57). 

4.  The  Nutrient  Artery  of  the  Femur  is  given  off  at  the  tendon  of 
insertion  of  the  pectineus. 

5.  An  Articular  branch,  of  slender  volume,  descends  between  the  vastus 
internus  and  adductor  magnus  to  the  stifle-joint. 

The  Femoral  Vein  ascends  in  close  company  with  the  artery,  and 
receives  branches  which  correspond  more  or  less  exactly  to  those  just 
described.  At  the  brim  of  the  pelvis  it  lies  posterior  to  the  artery,  and 
is  continued  upwards  as  the  external  iliac  vein. 

The  Anterior  Crural  Nerve  (Plate  13)  is  derived  from  the  lumbo- 
sacral plexus.  It  descends  between  the  psoas  magnus  and  parvus  ;  and 
passing  over  the  common  termiijation  of  the  iliacus  and  psoas  magnus, 
where  it  is  covered  by  the  sartorius,  it  splits  into  a  bundle  of  branches 
that  together  penetrate  between  the  vastus  internus  and  rectus  femoris 
to  supply  the  mass  of  the  quadriceps  extensor  cruris.  While  under 
cover  of  the  sartorius  it  gives  off  the  internal  saphena  nerve  already 
described. 

Directions. — The  pectineus,  adductor  parvus,  adductor  magnus,  and 
semimembranosus  muscles  are  now  to  be  cleaned  and  isolated.  These 
muscles  succeed  each  other  from  before  to  behind  in  the  order  named. 
Some  little  difficulty  may  be  experienced  in  finding  the  line  of  separa- 
tion between  the  two  adductoi'S,  but  a  reference  to  Plate  13  will  prove 
of  some  assistance.  Moreover,  the  fibres  of  the  small  adductor  are  of  a 
paler  colour  than  those  of  the  adductor  magnus. 


GO  THE  ANATOMY  OF  THE  HORSE. 

The  Pectixeus  (Plate  13).  This  muscle  has  a  distinctly  conical  form. 
It  lies  posterior  to  the  femoral  vessels,  and  the  profunda  artery  disap- 
pears beneath  it.  It  arises  from  the  brim  and  inferior  surface  of  the 
pubis,  and  it  is  there  penetrated  by  the  pubio-femoral  ligament,  from 
which  some  of  its  fibres  take  origin.  Its  tapering  point  is  inserted  into 
the  shaft  of  the  femur  in  the  neighbourhood  of  the  nutrient  foramen. 

Action. — It  adducts  the  limb,  and  flexes  the  hip. 

The  Adductor  Parvus  (Adductor  In-evis  of  Percivall)  (Plate  13)  is  situ- 
ated between  the  i)ectineus  and  the  great  adductor.  It  arises  from  the 
inferior  surface  of  the  pubis,  and  is  inserted  into  the  posterior  siirface  of 
the  femur  about  its  middle. 

Action. — It  is  an  adductor  and  outward-rotator  at  the  hip-joint. 

The  Adductor  Magnus  {Adductor  longtis  of  Percivall)  (Plate  13) 
arises  from  the  inferior  surftvce  of  the  ischium,  and  from  the  tendon  of 
origin  of  the  gracilis.  It  has  two  insertions,  between  which  the  femoral 
artery  passes.  1.  Its  deeper  fibres  are  inserted  into  the  posterior  surface 
of  the  femur,  on  a  quadrilateral  area  above  the  smooth  groove  in  which 
the  femoral  artery  rests.  2.  Its  more  superficial  and  longer  fibres  are 
inserted  into  the  forepart  of  the  supracondyloid  crest. 

Action. — It  is  an  adductor  at  the  hip. 

The  Semimembranosus  {Adductor  viagnus  of  Percivall)  (Plate  13).  This 
is  a  muscle  of  large  size.  It  arises  from  the  lower  surface  of  the  ischium, 
including  its  tuberosity,  and  by  a  small  slip  from  the  fiiscia  investing 
the  muscles  of  the  tail.  It  is  inserted  into  the  inner  condyle  of  the 
femur,  behind  the  tubercle  for  the  attachment  of  the  internal  lateral 
ligament  of  the  stifle. 

Action. — Commonly,  it  is  an  adductor  and  extensor  of  the  hip ;  but 
when  the  femur  is  fixed,  it  acts  as  a  lever  of  the  first  order,  and  assists 
in  rearing. 

Directions. — The  foregoing  muscles  must  now  be  cut  and  partially 
removed  as  follows  : — 

The  semimembranosus  is  to  be  cut  transversely,  an  inch  or  two  above 
its  insertion.  The  muscle  is  then  to  be  raised  upwards  from  the  semi- 
tendinosus,  on  which  it  rests ;  and  in  doing  this,  branches  of  nerves 
from  the  great  sciatic  will  be  found  entering  it  in  front.  The  central 
portion  of  the  muscle  may  then  be  removed,  leaving  a  few  inches  at  its 
origin.  The  other  muscles  must  be  served  in  the  same  way,  leaving 
only  short  portions  at  the  origin  and  insertion,  except  in  the  case  of  the 
adductor  parvus  and  upper  half  of  the  adductor  magnus,  whose 
common  insertion  into  the  back  of  the  femiu-  is  to  be  entirely  removed. 
Care  is  to  be  taken  of  the  femoral  artery  where  it  rests  on  the  bone,  and 
in  performing  the  dissection  it  will  be  well  to  refer  to  Plate  14  as  a 
guide.  In  reflecting  the  upper  portion  of  the  great  adductor,  a  branch 
of  the  obturator  nerve  will  be  found  entering  its  deej)  face,  after  having 


DISSECTION    OF    THE    POSTERIOR    LIMB.  61 

passed  through  the  obturator  externus  muscle.  Other  branches  of  the 
same  nerve  will  be  found  supplying  the  small  adductor  and  the  pecti- 
neus. 

The  Femoral  Artery  (Plate  14).  The  i-emaining  portion  of  this 
vessel  is  now  exposed  as  it  winds  round  behind  the  shaft  of  the 
femur,  leaving  its  impress  on  the  bone.  It  is  seen  passing  in  between 
the  heads  of  the  gastrocnemius  muscle,  at  which  point  it  takes  the 
name  of  jyopliteal.  In  this  part  of  its  coui'se  it  gives  off  only  one  vessel 
of  note — the  femoro-popliteal. 

The  Femoro-popliteal  Artery.  The  point  of  origin  of  this  branch 
marks  the  lower  limit  of  the  femoral  artery.  It  passes  backwards  in  a 
horizontal  direction,  and  penetrates  the  semitendinosus.  Near  its  origin 
it  gives  off  a  considerable  branch  which  ascends  behind  the  femur,  sup- 
plying the  biceps,  and  anastomosing  with  the  profunda.  Other  branches 
descend  from  it  to  the  gastrocnemius. 

PoPLiTiiAL  Lymphatic  Glands.  A  few  glands  will  be  found  on  the 
track  of  the  femoro-popliteal  artery  between  the  semitendinosus  and 
biceps  femoris  muscles. 

The  Profunda  or  Deep  Femoral  Artery.  In  the  preceding- 
stage  of  the  dissection  this  branch  of  the  femoral  was  seen  at  its 
origin.  It  passes  downwards  and  backwards,  between  the  adjacent 
edges  of  the  iliacus  and  obturator  externus,  and  under  cover  of  the 
pectineus  and  adductor  parvus.  Above  the  insertion  of  the  quadratus 
femoris  it  crosses  behind  the  femur,  where  its  terminal  branches  descend 
to  supply  the  biceps.  It  also  furnishes  collateral  branches  to  the 
pectineus,  gracilis,  and  adductors. 

Veins.  The  foregoing  arteries  are  accompanied  by  satellite  veins  of 
the  same  names. 

The  Quadratus  Femoris  (Plates  14  and  16).  This  is  a  somewhat  slender 
riband-shaped  muscle.  It  arises  from  the  lower  surface  of  the  ischium 
in  fi-ont  of  the  tuberosity,  and  it  becomes  inserted  into  an  oblique  line 
on  the  back  of  the  femur,  at  the  level  of  the  third  trochanter. 

Action. — It  is  an  extensor  and  outward-rotator  at  the  hip. 

The  Obturator  Externus  (Plate  14).  This  muscle,  which  is  coarsely 
fasciculated,  covers  the  obturator  foramen,  and  conceals  the  obturatoi- 
nerves  and  vessels  as  they  emerge  from  the  pelvis.  It  is  traversed  by 
two  branches  of  the  obturator  nei've,  the  posterior  of  which  is  for  the 
great  adductor,  while  the  anterior  splits  into  branches  for  the  supply 
of  the  small  adductor,  pectineus,  and  gracilis.  It  •  arises  from  the  lower 
surface  of  the  pubis  and  ischium,  and  is  inserted  into  the  trochanteric 


Action. — It  is  an  extensor  and  outward-rotator  at  the  hip. 
Directions. — The  nerves  which  emerge  from  the  obturator  externus 
should  be  traced  through  the  substance  of  that  muscle  to  their  oriaiu 


62  THE  ANATOMY  OF  THE  HORSE. 

from  the  obturator  nerve.  The  muscle  itself  may  then  be  removed  to 
expose  the  obturator  vessels  and  nerve. 

The  Obturator  Artery  (Plates  14  and  46).  This  vessel  begins  at  the 
pelvic  inlet  as  one  of  the  terminal  branches  of  the  internal  iliac.  It 
leaves  the  pelvis  by  the  obturator  foramen,  in  company  with  a  vein  and 
nerve  of  the  same  name.  At  its  point  of  emergence  it  is  covered  by 
the  obturator  externus,  and  it  passes  backwards  between  that  muscle 
and  the  bone,  and  then  curves  downwards  to  terminate  in  the  biceps 
and  semitendinosus.      It  gives  off  the  artery  of  the  coiyus  cavernosum. 

The  Obturator  Vein  passes  into  the  pelvis  by  the  obturator  foramen, 
and  aids  in  forming  the  internal  iliac  vein. 

The  Obturator  Nerve  is  a  branch  of  the  lumbo-sacral  plexus. 
Emerging  by  the  obturator  foramen,  it  divides  for  the  supply  of  the 
obturator  externus,  adductor  parvus,  adductor  magnus,  pectineus,  and 
gracilis  muscles. 

Directions. — In  this  stage  of  the  dissection  the  great  sciatic  nerve  is 
seen  in  its  course  downwards  through  the  thigh.  Its  examination  is 
more  conveniently  inidertaken  in  the  dissection  of  the  hip  and  outer 
aspect  of  the  thigh,  but  attention  may  also  be  given  to  it  here. 

The  Great  Sciatic  Nerve,  which  is  a  branch  of  the  lumbo-sacral 
plexus,  after  passing  through  the  hip  (see  Plate  16),  descends  in  the 
thigh,  behind  the  femur,  where  it  is  deeply  enclosed  between  the 
biceps  and  semitendinosus  outwardly,  and  the  semimembranosus  and 
great  adductor  inwardly.  Under  the  name  of  the  intei-nal  popliteal,  it 
passes  in  between  the  two  heads  of  the  gastrocnemius.  The  following 
branches  whose  points  of  origin  are  not  now  visible,  being  situated  in 
the  hip,  may  be  identified  by  reference  to  Plate  14: — (1)  Branches  to  the 
biceps,  semitendinosus,  and  semimembranosus ;  (2)  the  external  pop- 
liteal ;  (3)  the  external  saphenous.  The  last  two  will  be  again  seen  in 
the  dissections  of  the  hip,  thigh,  and  leg. 

Directions. — The  vastus  internus,  situated  at  the  front  of  the  thigh, 
should  now  be  examined.  It  is  a  division  of  the  great  muscular  mass 
termed  in  man  the  quadriceps  extensor  cruris,  Avhose  other  divisions — the 
rectus  femoris  and  vastus  extennis — will  be  dissected  with  the  outer 
aspect  of  the  thigh.  The  dissection  in  this  position  of  the  limb  will  be 
completed  by  an  examination  of  the  common  insertion  of  the  iliflcus 
and  psoas  magnus. 

The  Vastus  Internus  (and  Crureus*)  (Plates  13  and  14)  is  a  thick 
fleshy  muscle  whose  fibres  take  origin  from  the  internal  surface  and  inner 
half  of  the  anterior  surface  of  the  femur,  meeting  along  the  front  of  the 
femur  the  vastus  internus,  and  with  it  forming  a  groove  in  which  the 
rectus  femoris  rests.     Its  fibres  are  inserted  into  the  inner  ligament  of 

*  This  is  the  name  given  to  the  fourth  division  of  the  quadricejis  in  human  anatomy.  The  fibres 
that  represent  it  in  the  liorse  are  in  no  way  separable  from  tlie  inner  viistus.  Under  tlie  siime  name 
Percivall  describes  (inaccurately)  the  rectus  parvus. 


DISSECTION    OF    THE    POSTERIOR    LIMB.  63 

the  patella,  or  into  that  bone  along  with  the  other  divisions  of  the 
quadriceps. 

Action. — It  is  an  extensor  of  the  stifle-joint. 

Psoas  Magnus  and  Iliacus  (Plate  14).  Only  the  terminal  portion  of 
each  of  these  muscles  is  here  seen.  They  are  more  fully  displayed  in 
the  dissection  of  the  sublumbar  region,  where  the  psoas  magnus  arises 
from  the  last  two  ribs,  and  the  vertebrae  from  the  16th  dorsal  to  the  5th 
lumbar  (Plates  44  and  45).  The  iliacus  m-ises  from  the  iliac  surface  and 
external  angle  of  the  ilium,  and  from  the  sacro-iliac  ligament.  It  presents 
a  deep  groove  for  the  terminal  portion  of  the  psoas  magnus.  The  two 
muscles  pass  downwards  beneath  Poupart's  ligament,  and  have  a  com- 
mon insertion  into  the  small  (internal)  trochanter  of  the  femur. 

Action. — These  muscles  flex  the  hip-joint,  and  rotate  it  outwards. 

THE    HIP    AND    OUTER    ASPECT    OP    THE    THIGH. 

Position. — The  animal  should  be  suspended  in  imitation  of  the  natural 
standing  posture,  by  the  means  mentioned  at  page  8. 

Surface-marking. — A  prominent  feature  of  the  region  is  the  bony  pro- 
jection formed  by  the  external  angle  of  the  ilium  {angle  of  the  haunch). 
The  tuber  ischii  may  also  be  felt  by  pressing  deeply  at  the  point  of  the  hip. 
At  the  highest  part  of  the  croup  the  internal  angle  of  the  ilium  {angle 
of  the  croup)  may  be  felt,  and  in  the  middle  line  the  tips  of  the  sacral 
spines  are  subcutaneous.  In  a  lean  animal  a  number  of  grooves  are  seen 
marking  the  divisions  of  the  biceps  and  the  line  of  opposition  of  that 
muscle  with  the  semitendinosus  (Plate  15). 

Directions. — An  incision  through  the  skin  is  to  be  carried  along  the 
middle  line  from  the  root  of  the  tail  as  far  forwards  as  the  lumbar 
region,  where  a  transverse  incision  is  to  be  carried  outwards  and  down- 
wards as  far  as  the  level  of  the  angle  of  the  haimch.  Beginning  at  the 
middle  line  above,  the  dissector  is  to  reflect  the  skin  from  the  limb,  as 
far  as  the  middle  of  the  leg.  The  first  few  inches  of  the  skin  will 
require  to  be  raised  by  the  use  of  the  scalpel,  and  then  an  attempt  may 
be  made  to  tear  it  downwards  off  the  limb — a  method  which  will  show 
the  cutaneous  nerves  distinctly  without  further  dissection. 

Cutaneous  Nerves.  1.  Appearing  a  few  inches  from  the  middle  line, 
are  some  slender  branches  derived  from  the  sacral  nerves.  2.  Two  or 
three  branches  of  considerable  size,  derived  from  the  lumbar  nerves, 
pass  backwards  and  downwards  over  the  forepart  of  the  gluteal  region. 

3.  A  few  inches  below  the  point  of  the  hip  a  cutaneous  branch  derived 
from  one  of  the  posterior  gluteal  nei-ves  appears  from  between  the  biceps 
and  semitendinosus,  and  separates  into  a  number  of  radiating  filaments. 

4.  On  a  level  with  the  stifle-joint  the  p<^roneal-cutaneous  branch  of  the 
external  popliteal  nerve  comes  out  through  the  biceps,  and  is  distributed 
on  the  outer  side  of  the  leg. 


G4  THE  ANATOMY  OF  THE  HORSE. 

Directions. — The  dissector  shoiild,  in  the  next  place,  direct  his  atten- 
tion to  the  strong  fascia  covering  the  muscles  in  this  region,  after  which 
the  fascia  must  be  removed,  and  the  mnscles  cleaned  and  separated. 

Gluteal  Fascia  and  Fascia  Lata.  The  gluteal  fascia  forms  a  bluish- 
white  covering  over  the  muscles  of  the  hip,  and  by  its  deep  face  affords 
origin  to  many  fibres  of  the  supei*ficial  and  middle  gluteal  muscles.  It 
is  fixed  above  to  the  summits  of  the  sacral  spines  and  to  the  external 
angle  of  the  ilium,  and  between  these  points  it  is  continuous  forwards 
with  the  tendon  of  the  latissimus  dorsi.  It  is  prolonged  downwards  over 
the  muscles  of  the  thigh,  where  it  takes  the  name  of  the  fascia  lata. 
This  fascia  lata  receives  in  front  the  insertion  of  the  tensor  vaginae 
femoris  muscle,  and  it  should  not  be  removed  until  that  muscle  has  been 
examined.  It  forms  a  sheath  for  the  muscles  of  the  thigh,  and  is  pro- 
longed downwards  over  the  leg.  From  its  inner  face  a  septum  is  sent  in 
between  the  vastus  intcrnus  and  the  biceps,  to  join  the  tendon  of  the 
superficial  gluteal  muscle,  and  be  inserted  into  the  femur. 

The  Tensor  Vaginj^  Femoris  (Plate  15).  This  muscle  is  situated 
at  the  forepart  of  the  thigh,  in  front  of  the  superficial  gluteal  muscle, 
from  which  it  is  somewhat  difficult  to  separate  it.  It  arises  from  the 
external  angle  of  the  ilium,  and  it  is  inserted  into  the  fiiscia  lata. 

Action.- — It  flexes  the  hip-joint.  It  also  keeps  the  fascia  lata  tense, 
and  mechanically  aids  in  keeping  the  stifle-joint  extended. 

Directions. — The  gluteal  fascia  and  the  fascia  lata  are  now  to  be 
removed.  It  is  a  matter  of  some  difficulty  to  remove  the  former,  as  its 
deep  face  has  the  muscular  fibres  taking  origin  from  it,  and  these  are 
therefore  exposed  with  a  rough  surfjice  when  it  is  removed. 

The  Superficial  Gluteus  (Glntetts  extemus  of  Percivall,  part  of  the  glu- 
teus maximus  of  human  anatomy)  (Plate  15).  The  outline  of  this  muscle 
is  not  distinctly  recognisable  until  the  gluteal  fascia  has  been  removed. 
It  is  then  seen  to  have  some  resemblance  to  the  letter  V,  having  in  its 
upper  border  an  indentation  that  divides  it  into  an  anterior  and  a 
posterior  branch.  The  anterior  branch  arises  from  the  external  angle 
of  the  iliiim ;  the  posterior  from  the  gluteal  fascia.  Both  converge 
to  a  common  tendon,  which  is  inserted  into  the  third  trochanter  of  the 
femur  (trochanter  minor  extemus).  From  the  posterior  branch  of  the 
miiscle  an  aponeurotic  layer  passes  backwards  beneath  the  biceps,  to 
be  inserted  into  the  sacro-sciatic  ligament  and  the  tuber  ischii. 

Action. — It  is  an  abductor  at  the  hip-joint. 

The  Biceps  Femoris  (Plate  15).  This  is  one  of  the  largest  muscles 
in  the  body.  It  arises  from  the  sacral  spines,  the  fascia  enveloping  the 
muscles  of  the  tail,  the  sacro-sciatic  ligament,  the  tuber  ischii,  and  the 
gluteal  fascia.  Inferiorly  it  has  three  divisions,  one  of  which  is  inserted 
into  the  anterior  surface  of  the  patella,  a  small  synovial  bursa  being 
interposed  between  the  tendon  and  the  bone,  another  into  the  tibial 


DISSECTION    OF    THE    POSTERIOR    LIMB.  65 

crest,  and  the  third  into  the  fascia  of  the  leg.  Besides  these,  the 
muscle  has  an  insertion  into  the  circular  mark  behind  the  third 
trochanter  of  the  femur,  by  means  of  a  fibrous  band  detached  from  the 
deep  surface  of  the  muscle. 

Action. — The  anteiior  half  of  the  muscle,  in  virtue  of  its  attachment 
to  the  patella,  is  an  extensor  of  the  stifle,  and  an  abductor  at  the  hip. 
The  posterior  half  of  the  muscle,  with  its  insertions  into  the  tibia  and 
fascia  of  the  leg,  is  a  flexor  and  an  outward-rotator  at  the  stifle.  When 
the  stifle-joint  is  kept  extended,  the  lower  end  of  the  muscle  becomes  its 
fixed  point,  and  it  then  extends  the  pelvis  on  the  femur,  and  aids  in 
rearing. 

The  Semitbndinosus  (Plate  15).  This  muscle  is  placed  at  the 
posterior  border  of  the  hip  and  thigh,  where  it  occvipies  a  position 
between  the  last-described  muscle  and  the  semimembranosus.  The 
muscle  is  bifid  superiorly,  where  it  arises  by  one  division  from  the 
sacral  spines  and  sacro-sciatic  ligament,  and  by  another  and  shorter 
branch  from  the  tuber  ischii.  Inferiorly  it  has  a  flat  tendon,  which  is 
inserted  into  the  tibial  crest,  and  whose  posterior  border  blends  with 
the  fascia  of  the  leg. 

Action. — To  flex  the  stifle  and  rotate  the  leg  inwards.  When  the 
stifle  is  fixed,  it  can  aid  in  rearing. 

The  biceps  femoris  and  semitendinosus  muscles  represent,  apparent^, 
the  muscles  of  the  same  name  in  man,  plus  portions  of  the  gluteus 
maximus.  Percivall  describes  the  semitendinosus  as  representing  also 
the  semimembranosus  of  man. 

Directions. — The  biceps  mvist  be  carefully  severed  at  its  origin,  and 
pulled  downwards.  This  will  expose  the  aponeurotic  layer  that  passes 
beneath  it  from  the  superficial  gluteus.  A  branch  from  the  posterior 
gluteal  nerves  should  be  found  entering  the  last-named  muscle  by  turn- 
ing forwards  round  the  middle  gluteus.  Both  branches  of  the  super- 
ficial gluteus  should  then  be  thrown  downwards  in  order  to  fully 
expose  the  next  muscle. 

The  Middle  Gluteus  (Glnietis  maximus  of  Percivall)  (Plate  15)  is  a 
muscle  of  great  size  and  strength.  It  was  partly  exposed  before  the 
removal  of  the  superficial  muscle.  The  fibres  of  the  muscle  arise  from 
the  aponeurosis  of  the  common  mass  of  the  loins  (longissimus  dorsi), 
from  the  gluteal  surface  of  the  ilium,  from  the  two  ilio-sacral  and  the 
sacro-sciatic  ligaments,  and  from  the  gluteal  fascia.  It  has  three  distinct 
and  constant  insertions:  1.  By  a  tendon,  into  the  swnmit  of  the  great 
trochanter.  2.  By  another  tendon,  which  plays  over  the  convexity  of  the 
same  trochanter  by  means  of  a  synovial  bursa,  and  becomes  inserted  into 
the  crest.  3.  By  a  triangular  fleshy  slip,  into  the  back  of  the  trochan- 
teric ridge. 

Action. — To  extend  and  abduct    the  hip.     In  the  former  of  these 

F 


66  THE    A>'ATOMY    OF    THE    IIOKSE. 

actions,  when  the  limb  is  free  to  move,  the  femur,  and  with  it  the 
whole  limb,  is  carried  backwards  ;  but  when  the  femur  is  fixed,  it  raises 
the  trunk,  as  in  rearing. 

Directions. — The  last-described  muscle  must  be  removed  in  order  to 
expose  the  deep  gluteus  and  the  other  structures  which  it  covers.  A 
deep  incision  shoidd  be  made  through  the  muscle  along  the  crest  of  the 
ilium,  and  the  muscle  is  to  be  turned  down  by  severing  its  fibres  at 
their  origin.  Care  must  be  taken,  in  doing  this,  to  avoid  cutting  the 
subjacent  deep  gluteus,  whose  fibres  may  be  recognised,  as  soon  as  they 
are  reached,  by  their  insertion  uithin  the  great  trochanter.  A  reference 
to  Plate  16  may  here  be  useful.  The  semitendinosus  is  to  be  turned 
down  in  the  same  manner,  by  severing  its  superior  attachments  ;  and 
this  muscle  and  the  biceps  may  be  removed  to  the  extent  shown  in  the 
Plate.  In  performing  this  dissection,  the  gluteal  nerves  and  vessels  and 
the  ischiatic  vessels  are  unavoidably  severed,  but  a  look-out  should 
be  kept  for  these,  and  they  should  be  cut  about  the  points  shown  in 
the  figure. 

The  Gluteal  Artery  (Plate  16)  is  a  branch  given  off  from  the 
internal  iliac  within  the  pelvis.  After  a  very  short  course  it  splits  into 
several  branches,  which,  emerging  by  the  great  sacro-sciatic  opening, 
are  distributed  to  the  gluteal  muscles. 

The  Ischiatic  Artery  (Plate  16)  is  one  of  the  terminal  branches 
of  the  lateral  sacral  artery,  which,  again,  is  a  collateral  branch  of  the 
internal  iliac.  It  perforates  the  sacro-sciatic  ligament  near  the  edge  of 
the  sacrum,  and  is  distributed  in  the  biceps  and  semitendinosus. 

Veins  of  the  same  names  accompany  the  foregoing  arteries. 

The  Internal  Pudic  Artery.  The  dissection  of  this  artery  belongs 
to  another  region,  but  the  vessel  is  generally  visible  here  in  a  part  of 
its  course.  A  few  inches  of  it  are  represented  in  Plate  16,  as  showing 
faintlv  thi'ough  the  textui-e  of  the  sacro-sciatic  ligament.  It  is  described 
at  page  342. 

The  Gluteal  Nerves  (Plate  16).  These  nerves,  which  are  derived 
from  the  lumbo-sacral  plexus,  issue  from  the  great  sacro-sciatic  opening 
in  company  with  the  gluteal  vessels  and  the  great  sciatic  nerve.  They 
consist  of  an  anterior  and  a  posterior  set. 

The  Anterior  gluteal  nerves  are  three  or  four  in  number.  One  of  them 
passes  downwards  and  forwards  between  the  middle  and  internal 
gluteal  muscles,  to  reach  the  tensor  vaginae  femoris  and  anterior  division 
of  the  superficial  gluteus.  Another  branch  passes  downwards  over  the 
deep  gluteus,  to  which  it  is  distributed.  One  or  two  other  branches 
supply  the  middle  gkiteus. 

The  Posterior  gluteal  nerves  are  two  in  number — an  upper  and  a 
lower.  The  upper  nerve  passes  backwards  on  the  sacro-sciatic  liga- 
ment; and   after   giving   branches    to    the   posterior   division  of  the 


DISSECTION    OP    THE    POSTERIOR    LIMB.  K)  t 

superficial  gluteus,  and  to  the  posterior  fleshy  slip  of  the  middle 
gluteus,  it  enters  the  biceps  fenioris.  The  loivei-  nerve,  passing  down- 
wards and  backwards,  divides  into  an  outer  and  an  inner  branch  ;  the 
former,  turning  over  the  tuber  ischii  on  its  outer  side,  becomes  cutane- 
ous at  the  back  of  the  thigh  about  four  or  five  inches  below  the  tuber ; 
the  latter,  after  giving  twigs  to  the  semitendinosus,  joins  a  branch  from 
the  internal  pudic  nerve  to  be  distributed  to  the  perineal  structures. 

The  Great  Sciatic  Nerve  (Plate  16).  This  is,  at  its  point  of 
origin,  the  largest  nerve  in  the  body.  It  is  furnished  by  the  lumbo- 
sacral plexus,  and  appears  at  the  great  sacro-sciatic  opening  as  a  broad 
riband.  In  its  downward  course  in  the  hip  it  is  covered  by  the  middle 
gluteus,  and  rests  in  succession  on  the  sacro-sciatic  ligament,  the 
gluteus  internus,  the  gemelli  and  common  tendon  of  the  obturator 
internus  and  pyriformis,  and  the  quadratus  femoris.  In  the  thigh  it  is 
included  between  the  biceps  and  semitendinosus  outwardly,  and  the 
semimembranosus  and  great  adductor  inwardly.  The  trunk  of  the  nerve 
is  continued  as  the  internal  popliteal  nerve  between  the  two  heads  of 
the  gastrocnemius,  where  it  will  be  followed  in  the  dissection  of  the 
leg.  It  gives  off  in  succession  the  following  branches  : — 1.  A  nerve  for 
the  supply  of  the  obturator  internus,  pyriformis,  gemelli,  and  quadratus 
femoris.  This  slender  branch  is  given  off  about  midway  between  the 
great  and  small  sciatic  openings,  and  it  descends  at  the  posterior  border 
of  the  parent  trunk,  or  between  that  and  the  ligament.  The  nerves  to 
the  quadratus  and  gemelli  may  arise  from  the  sciatic  independently, 
and  the  branch  to  the  first  of  these  muscles  passes  under  the  gemelli 
and  the  common  tendon  of  the  pyriformis  and  obturator  internus. 
2.  The  external  popliteal  nerve  is  a  large  branch  that  separates  from  the 
great  sciatic  about  the  level  of  the  small  sacro-sciatic  opening ;  and, 
descending  in  front  of  the  parent  nerve,  it  passes  between  the  biceps 
and  the  outer  head  of  the  gastrocnemius,  where  it  will  be  followed  at  a* 
later  stage.  The  2>^'>'oneal  cutaneous  branch  of  this  nerve  has  already 
been  seen  perforating  the  lower  part  of  the  biceps,  at  the  level  of  the 
stifle.  3.  A  branch  that  divides  to  supply  the  semimembranosus  and 
lower  portions  of  the  biceps  and  semitendinosus  (Plate  14).  4.  The  ex- 
ternal saphenous  nerve,  which  will  be  followed  in  the  dissection  of  the  leg. 

Directions. — The  great  sciatic  nerve  should  now  be  cut  at  the  upper 
border  of  the  gluteus  internus,  and  turned  downwards  with  its  branches. 
The  gluteus  internus,  and  the  common  tendon  of  the  pyriformis  and 
obturator  internus,  together  with  the  gemelli,  should  be  carefully 
cleaned  and  defined ;  and  to  facilitate  this,  the  limb  should  be  rotated 
inwards  as  far  as  possible,  by  pulling  the  point  of  the  hock  outwards. 
This  will  put  these  muscles  on  the  stretch. 

The  Deep  Gluteus,  or  gluteus  internus  (Plate  16,  and  fig.  2),  is  placed 
above  the  hip-joint,  in  immediate  contact  with  the  capsular  ligament. 


68 


THE  ANATOMY  OF  THE  HORSE. 


It  is  a  comparatively  small  muscle,  with  coarse  fasciculi  having  a  slightly 
spiral  direction.  It  fu-ises  from  the  rough  lines  on  the  gluteal  surface  of 
the  shaft  of  the  ilium  just  above  the  cotyloid  cavity,  and  from  the  supra- 
cotyloid  ridge  (superior  ischiatic  spine).  It  is  inserted  to  the  inner  side 
of  the  convexity  of  the  great  trochanter. 

Action. — It  is  an  abductor  and  inward-rotator  at  the  hip-joint. 

The  Obturatou  Ixternus  and  the  Pyriformis  (Plate  16,  and  fig.  2) 
are  two  muscles  arising  within  the  pelvis,  the  former  taking  ori(/in  from 
the  bone  around  the  obturator  foramen,  and  the  latter  from  the  pelvic 
surface  of  the  ilium.  They  have  a  common  tendon,  which  emerges 
from  the  pelvis  by  the  lesser  sacro-sciatic  opening,  where  it  plays  over 
a  smooth  portion  on  the  external  border  of  the  ischium.  The  tendon 
is  inserted  into  the  trochanteric  fossa. 

Action. — To  produce  outward  rotation  at  the  hip. 

The  Gemelli.  In  Plate  16  a  bundle  of  muscular  fibres  is  seen  at 
each   edge  of  the  above-mentioned  common  tendon.     If  this  common 


Fig.  2. 

Muscles  of  the  Tail,  deep  Muscles  ok  the  Hip,  and  Pelvic  Lujaments  (Chaui-eav.). 

1.  Erector  coccygie ;  2.  Curvator  coccygis ;  3.  Depressor  coccygis ;  4.  Compressor  coccygis 
.1.  Deep  ghiteus ;  6.  Rectus  parvus  ;  7.  Common  tendon  of  obturator  internus  and  pyriformis 
S.  Gemelli ;  !».  Accessory  fasciculus  of  the  fame  ;  10.  Qtiailratus  femoris;  11.  Sacro-sciatic  ligament 
12.  Great  sacro-sciatic  foramen  ;  13.  Superior  ilio-sacral  ligament ;  14.  Inferior  ilio-sacral  ligament. 

tendon  be  cut  where  it  appears  at  the  lesser  sciatic  opening,  and  raised 
outwards,  what  previously  seemed  two  distinct  muscular  bundles  will 
now  be  seen  to  be  the  edges  of  a  single  flattened  muscle,  which  arues 
from  the  ischium  below  and  at  the  edges  of  the  smooth  surface  for  the 


DISSECTION    OF    THE    POSTERIOR    LIMB.  by 

passage  of  the  common  tendon,  and  becomes  inserted  in  common  with 
that  tendon.* 

Action. — The  same  as  the  two  preceding  muscles. 

Directions. — The  tensor  vaginae  femoris  and  the  ghiteus  internus 
should  now  be  cut  away,  care  being  taken  not  to  injure  the  capsular 
ligament,  on  which  the  latter  muscle  rests.  The  rectus  femoris,  vastus 
externus,  and  rectus  parvus  are  then  to  be  dissected.  The  last- 
mentioned  muscle  will  be  found  by  dissecting  deeply  into  the  upper  part 
of  the  interstice  between  the  other  two  muscles,  and  at  the  same  point 
the  iliaco-femoral  artery  will  be  found. 

The  Iliaco-femoral  Artery  is  one  of  the  terminal  branches  of  the 
internal  iliac  (Plate  48).  It  comes  out  between  the  iliacus  and  the 
shaft  of  the  ilium,  and  penetrates  between  the  rectus  femoris  and 
the  vastus  externus. 

The  Rectus  Femoris  (Plate  18)  arises  by  two  heads — one  from  each 
of  the  depressions  on  the  shaft  of  the  ilium,  above  and  in  front  of  the 
cotyloid  cavity.  The  central  portion  of  the  muscle  is  thick  and  fleshy, 
and  rests  in  a  groove  formed  by  the  twoVasti,  with  which  it  is  confounded 
at  its  lower  extremity.  It  is  inserted  into  the  anterior  face  of  the 
patella. 

Action. — To  flex  the  hip-joint  and  extend  the  stifle. 

The  Vastus  Externus  (Plate  18)  arises  from  the  outer  surface  of  the 
femur,  and  from  the  outer  half  of  the  anterior  surface  of  the  same  bone. 
Its  fibres  become  inserted  along  Avith  the  rectus  femoris  into  the  patella. 

Action. — -To  extend  the  stifle. 

The  Rectus  Parvus  (fig.  2)  is  very  slender  when  compared  with 
the  muscles  between  which  it  is  placed,  being  about  the  thickness  of 
a  humaii  finger.  It  arises  from  the  ilium,  external  to  the  pit  from 
which  the  outer  head  of  the  rectus  femoris  takes  origin.  Passing  in 
front  of  the  capsular  ligament  of  the  hip-joint,  to  which  it  adheres, 
it  insinuates  itself  between  the  two  vasti  muscles,  and  is  inserted 
into  the  anterior  surface  of  the  femur. 

Action. — The  muscle  is  of  too  slender  a  size  to  exert  any  appreci- 
able flexor  action  on  the  hip-joint,  and  probably  its  function  is  to  raise 
the  capsular  ligament  during  flexion  of  the  joint.  (See  footnote, 
page  62.) 

Directions. — The  dissector  is  now  in  a  position  to  detach  the  limb 
from  the  trunk,  and  this  should  be  done  by  cutting  through  the  bone 
and  soft  structures,  below  the  level  of  the  internal  trochanter.  It  is 
necessary  to  make  the  section  at  this  point,  in  order  to  leave  the  hip- 
joint  and  the  common  insertion  of  the  iliacus  and  psoas  magnus  intact 
for  examination  by  the  dissector  of  the  abdomen  and  pelvis.     The  limb 

*  In  man  this  muscle  consists  of  two  separate  slii)s,  and  from  this  disposition  it  is  named.  I  do 
not  hesitate  to  give  it  the  same  designation  here,  although  I  have  never  found  it  double  as  it  is 
usually  described. 


<U  THE  ANATOMY  OF  THE  UoRSE. 

having  been  removed,  it  should  be  placed  on  a  table,  and  the  cut 
muscles  connected  with  it  may  be  completely  cut  away  after  they 
have  been  identified.  In  doing  this,  a  better  opportunity  will  be 
afforded  to  observe  accurately  the  insertion  of  each  muscle.  In  remov- 
ing the  lower  portion  of  the  biceps,  particular  care  must  be  taken 
not  to  cut  the  external  popliteal  and  external  saphenous  nerves, 
which  are  included  between  it  and  the  outer  licad  of  the  gastrocnemius 
(Plate  18). 


Surface-marking. — The  bones  of  the  leg  are  clothed  by  muscles 
except  at  the  inner  side  of  the  limb,  where  the  tibia  is  subcutaneous. 
This  unprotected  area  of  bone  corresponds  to  the  shin  in  man.  The 
superficial  muscles  of  the  region  (see  Plates  17  and  18)  form  promi- 
nences more  or  less  distinct,  especially  in  the  neighbourhood  of  the  hock, 
where  the  various  tendons  stand  out  distinctly  during  the  movements 
of  the  living  animal. 

Position. — In  the  further  dissection  of  the  limb,  it  may  be  placed  on  a 
clean  table,  and  laid  on  either  side  as  may  be  convenient ;  or  a  cord  may 
be  passed  round  the  femur,  and  the  limb  suspended  at  such  a  height  as 
just  to  permit  the  hoof  to  come  into  contact  with  the  table.  This  latter 
method  has  the  advantage  of  keeping  the  jjart  clean ;  and  while 
dissection  is  being  carried  on,  the  leg  may  be  steadied  in  any  position  by 
an  assistant. 

Directions. — An  incision  through  the  skin  is  to  be  canned  down  the 
middle  line  of  the  limb  on  its  inner  side,  and  terminated  a  few  inches 
below  the  hock,  where  a  circular  incision  may  be  carried  round  the 
limb.  The  whole  of  the  skin  above  the  circular  incision  is  then  to  be 
removed,  and  the  cutaneous  nerves  and  vessels  of  the  region  are  to  be 
examined. 

The  Internal  Saphena  Vein.  On  the  inner  side  of  the  leg,  above 
the  hock,  two  venous  branches  will  be  seen  to  converge  and  unite  to 
form  the  internal  saphena  vein,  Avhich  is  continued  up  the  leg  to  the 
thigh,  where  it  has  already  been  dissected.  The  vessels  by  whose  union 
the  main  vein  is  formed,  are  the  upward  continuations  of  the  internal 
and  external  metatarsal  veins.  Slender  branches  of  the  saphena  ai'tery 
accompany  these  veins. 

The  External  Saphena  Vein,  This  vessel  begins  at  the  hock,  where 
it  communicates  with  the  internal  saphena  vein,  and  with  the  posterior 
tibial  vein.  It  ascends  at  the  outer  side  of  the  gastrocnemius  tendon, 
and,  passing  between  that  muscle  and  the  bice])s,  it  empties  itself  into 
the  femoro-popliteal  vein. 

Cutaneous  Nerves.  1.  The  ramifications  of  the  internal  .'saphenous 
nerve  cover  the  inside  of  the  leg,  and  descend  over  the  inside  of  the 


DISSECTION    OP    THE    POSTERIOR    LIMB.  71 

hock.  2.  The  external  saphe)ioits  nerve  (Plate  18)  is  a  branch  of  the 
great  sciatic.  It  descends  over  the  outer  head  of  the  gastrocnemius, 
where  it  is  covered  by  the  biceps,  and  is  reinforced  by  a  branch  from 
the  external  popliteal  (or  from  the  peroneal  cutaneous  division  of  that 
nerve).  It  then  continues  to  descend,  lying  in  company  with  the  vein  of 
the  same  name,  in  front  of  the  outer  edge  of  the  gastrocnemius  tendon ; 
and  passing  over  the  hock,  it  is  distributed  to  the  skin  on  the  outer 
side  of  the  metatarsus.  3.  The  cutaneous  termination  of  the  imismlo- 
cutaneous  division  of  the  external  popliteal  nerve  (Pkite  18)  pierces  the 
deep  fascia  on  the  outer  side  of  the  limb  at  the  lower  third  of  the  leg ; 
and,  passing  over  the  hock,  it  is  distributed  to  the  skin  on  the  front  of 
the  metatarsus. 

Deep  Fascia  of  the  Leg.  This  forms  a  close-fitting,  fibrous 
envelope  to  the  muscles  of  the  region.  Its  inner  face  furnishes  septa 
that  pass  in  between  the  muscles ;  and  over  the  inner  surface  and  crest 
of  the  tibia,  it  is  adherent  to  the  bone.  Above  it  is  continuous  with  the 
fascia  lata  and  tendons  of  the  gracilis  and  semitendinosus  on  the  inside, 
and  with  the  tendon  of  the  biceps  on  the  outside.  As  it  passes  over  the 
hock  it  becomes  thinner,  and  is  continuous  with  the  fascia  of  the  meta- 
tarsal region. 

Directions — The  muscles  on  the  back  of  the  leg  may  now  be  dissected, 
the  vessels  and  nerves  shown  in  Plates  17  and  18  being  at  the  same 
time  carefully  preserved.  By  a  reference  to  Plate  18,  the  student 
should  note  the  position  of  the  small  salens  muscle,  so  as  to  avoid  its 
removal  with  the  fascia. 

The  Gastrocnemius  (Plates  17  and  18).  At  its  origin  this  muscle 
consists  of  two  distinct  fleshy  heads,  which  terminate  in  a  single  inferior 
tendon.  The  outer  head  arises  from  the  outer  lip  of  the  supracondyloid 
fossa  of  the  femur,  the  inner  head  from  the  supracondyloid  crest.  The 
cord-like  tendon  is  joined  by  that  of  the  soleus,  and  is  inserted  into  the 
back  part  of  the  summit  of  the  os  calcis.  When  the  hock  is  strongly  flexed, 
the  tendon  for  an  inch  or  two  above  its  insertion  rests  on  the  forepart 
of  the  summit,  and  a  small  synovial  bursa  is  here  interposed  between 
the  tendon  and  the  bone.  The  tendon  of  the  perforatus  is  at  first 
beneath  that  of  the  gastrocnemius ;  but,  passing  to  the  inner  side,  it 
places  itself  superficial  to  the  latter,  which  it  completely  covers  at  the 
summit  of  the  os  calcis.  In  thus  changing  positions,  the  two  tendons 
form  a  half  twist,  and  indent  each  other  like  the  strands  of  a  rope. 
This  tendon  of  the  gastrocnemius  corresponds  to  the  firm  tendon 
extending  upwards  from  the  human  heel,  and  known  as  the  tench  Achillis. 

Action. — To  extend  the  hock-joint. 

The  Soleus  (Plate  18).  In  British  veterinary  text-books  this  muscle 
is  erroneously  termed  ^:)^«?ito7'is.  It  is  a  small  muscle  of  delicate 
texture,   and  it  is  often  partially  or  entirely  removed  in   cleaning  the 


i  2  THE  ANATOMY  OF  THE  HORSE. 

gastrocnemius.     It  arises  from  the  head  of  the  fibula,  and  its  tendon 
joins  that  of  the  preceding  muscle,  which  it  assists  in  extending  the  hock. 

Directions. — The  inner  head  of  the  gastrocnemius  is  to  be  severed  at 
its  origin,  and  turned  downwards  in  the  manner  shown  in  Plate  17. 

The  Superficial  Flexor  of  the  digit  (flexor  perforatus)  (Plates  17  and 
18)  is  remarkable  in  that,  throughout  nearly  the  whole  of  its  extent,  it 
exists  as  a  strong  tendinous  cord  with  a  spanng  admixture  of  muscular 
fibres  at  its  upper  fifth  only.  It  arises  from  the  bottom  of  the  supra- 
condyloid  fossa ;  and,  winding  round  the  gastrocnemius  tendon  in 
the  manner  already  described,  it  gains  the  summit  of  the  os 
calcis,  over  the  extreme  posterior  portion  of  which  it  plays  by 
means  of  a  synovial  bursa.  At  the  os  calcis  it  detaches  on  each  side  a 
slip  to  be  inserted  into  the  bone.  It  is  continued  downwards  in  the  meta- 
tarsal and  digital  regions  in  the  same  manner  as  the  perforatus  of  the 
fore  limb,  becoming  finally  inserted  by  a  bifid  termination  into  the 
second  phalanx.  In  front  of  the  tendons  of  the  superficial  flexor  and 
gastrocnemius  there  will  be  noticed  a  strong  fibrous  band,  which  is 
united  to  these  muscles  above,  and  inserted  into  the  os  calcis  below, 
while  laterally  it  is  continuous  with  the  deep  fascia  of  the  leg. 

Action. — It  flexes  successively  the  pastern  and  fetlock  joints  ;  and,  by 
its  insertion  into  the  os  calcis,  it  is  also  an  extensor  of  the  hock-joint. 
It  also  plays  an  important  part  in  mechanically  maintaining  the  hock 
in  a  state  of  extension  so  long  as  the  hip  and  stifle  joints  are  kept 
extended  by  muscular  contraction. 

The  flexor  perforatus  of  the  horse  is  represented  in  man  by  two 
distinct  muscles — the  pl'-intans  and  the  flexor  brevis  digitorum. 

Directions. — The  deep  layer  of  muscles  at  the  back  of  the  leg  consists 
of  the  popliteus,  the  flexor  perforans,  and  the  flexor  accessorius  ;  and 
these  should  now  be  examined  as  far  as  possible  without  disturbance  of 
the  vessels  and  nerves.  The  superficial  muscles  must  therefore,  in  the 
meantime,  be  allowed  to  remain  in  position. 

The  Popliteus  (Plate  17).  This  muscle  is  placed  immediately  behind 
the  stifle-joint,  whose  posterior  ligament  it  covers.  It  arises  by  a  tendon 
from  the  lower  and  most  anterior  of  the  two  pits  situated  on  the  outer 
side  of  the  external  condyle  of  the  femur.  (The  other  pit  is  for  the 
attachment  of  the  external  lateral  ligament  of  the  stifle,  the  ligament 
concealing  the  origin  of  the  tendon.)  The  tendon  is  partly  invested  by 
the  synovial  membrane  of  the  joint,  and  plays  round  the  external  semi- 
lunar cartilage,  and  over  the  articular  surfiice  of  the  tibia.  The  fibres 
of  the  muscle  have  an  oblique  direction  downwards  and  inwards,  and  are 
inserted  into  the  comparatively  smooth  triangular  aix-a  at  the  upper  part 
of  the  posterior  surfiice  of  the  tibia,  and  into  the  inner  edge  of  the  bone 
at  the  same  level.  The  terminal  portion  of  the  popliteal  artery  is 
concealed  by  the  muscle. 


DISSECTION    OF    THE    POSTERIOR    LIMB.  /  6 

Action. — It  flexes  the  stifle,  and  to  a  slight  extent  rotates  it  inwards. 

The  Deep  Flexor  of  the  digit  (flexor  perforans)  (Plates  17  and  18). 
This  muscle  is  indistinctly  divided  into  an  outer  and  an  inner  division, 
the  former  being  the  larger  of  the  two.  It  arises  from  the  ridged  area 
on  the  posterior  surface  of  the  tibia,  from  the  external  tuberosity  at  the 
upper  end  of  the  same  bone,  from  the  fibula,  and  from  the  iutei'osseous 
membrane  uniting  the  two  bones.  At  the  lower  third  of  the  tibia  the 
muscular  divisions  are  succeeded  by  tendons,  which  soon  unite  ;  and  the 
single  tendon  thus  formed  glides  through  the  tarsal  sheath  at  the  inner 
side  of  the  os  calcis,  and  then  descends  at  the  back  of  the  metatarsus 
and  digit,  to  be  inserted  into  the  solar  surface  of  the  os  pedis,  in  a  man- 
ner exactly  similar  to  the  flexor  perforans  of  the  fore  limb.  Like  that 
muscle,  it  receives,  at  the  upper  part  of  the  metatarsus,  a  reinforcing 
or  check  band — the  subtarsal  ligament,  which  is  the  downward  continua- 
tion of  the  posterior  tarso-metatarsal  ligament  of  the  hock.  This  band 
is  not  so  strong  as  the  subcarpal  ligament  of  the  fore  extremity. 

Action. — It  flexes  successively  from  below  upwards  the  interphalangeal 
joints  and  the  fetlock,  and  finally  extends  the  hock. 

The  Tarsal  Sheath,  through  which  the  tendon  passes  at  the  inner 
side  of  the  back  of  the  hock,  is  formed  outwardly  by  the  grooved  surface 
of  the  OS  calcis,  in  front  by  the  posterior  ligament  of  the  tibio-tarsal 
articulation  and  by  the  posterior  tarso-metatarsal  ligament,  and  it  is 
completed  inwardly  by  a  fibrous  arch  that  converts  the  groove  into  a 
complete  canal.  An  extensive  synovial  membrane  here  invests  the 
tendon  and  lines  the  passage,  extending  upwards  for  a  few  inches  at 
the  lower  exti'emity  of  the  tibia,  and  downwards  below  the  middle  of 
the  metatarsus.  A  dropsical  condition  of  this  synovial  sac  gives  rise 
to  the  condition  termed  "thorough-pin." 

The  Flexor  Accessorius  (Plate  17)  is  a  somewhat  slender  muscle 
extending  obliquely  downwards  at  the  back  of  the  leg,  between  the 
popliteus  and  the  perforans.  It  arises  from  the  back  of  the  external 
tuberosity  at  the  head  of  the  tibia.  Its  tendon,  which  begins  at  the 
lower  third  of  the  leg,  descends  first  in  a  groove  on  the  deep  flexor,  and 
then  through  a  synovial  passage  at  the  inner  side  of  the  tarsus,  and 
finally  blends  with  the  tendon  of  the  deep  flexor  at  the  back  of  the 
metatarsus. 

Action. — To  assist  the  deep  flexor. 

Directions. — The  vessels  and  nerves  of  the  region  must  now  be  noticed, 
and  it  will  be  convenient  to  begin  with  the  latter. 

The  Internal  Popliteal  Nerve  (Plates  17  and  18)  is  the  continuation 
of  the  great  sciatic.  It  passes  in  between  the  two  heads  of  the  gastro- 
cnemius muscle,  follows  for  a  short  distance  the  posterior  border  of  the 
perforatus,  and  at  the  level  of  the  lower  border  of  the  popliteus  it  is 
continued  under  the  name  of  the  posterior  tibial  nerve.      The  nerve  fur- 


74  THE  ANATOMY  OF  THE  HOUSE. 

nishcs  branches  to  all  the  muscles  at  the  back  of  the  leg,  viz.,  both 
heads  of  the  gastrocnemius,  the  sole  us,  the  perforatus,  the  popliteus, 
the  perforans,  and  the  flexor  accessorius.  The  branch  to  the  soleus  gains 
its  muscle  by  passing  between  the  popliteus  and  tiic  outer  head  of  the 
gastrocnemius. 

The  Posterior  Tibial  Nerve  (Plate  17)  continues  the  internal 
popliteal.  It  is  at  first  deeply  placed  beneath  the  inner  head  of  the 
gastrocnemius,  where  it  crosses  the  perforatus.  Becoming  more  super- 
ficial by  emerging  from  beneath  the  first-named  muscle,  it  descends  on 
the  inner  side  of  the  leg,  in  front  of  the  tendo  Achillis,  being  covered  by 
the  deep  foscia  of  the  leg.  At  the  tai'sus  it  bifurcates  to  form  the 
external  and  internal  plantar  nerves.  These  accompany  the  perforans 
tendon  through  the  tarsal  sheath,  and  are  continued  through  the  meta- 
tarsal and  digital  regions  like  the  corresponding  nerves  of  the  fore  limb. 
The  only  collateral  branches  of  the  posterior  tibial  nerve  arc  slender 
cutaneous  filaments,  one  of  which  is  shown  in  Plate  1 7,  descending  over 
the  inner  side  of  the  hock. 

The  External  Popliteal  Nerve  and  the  External  Saphenous  Nerve 
cross  the  external  head  of  the  gastrocnemius  on  its  outer  side  (Plate  18). 
The  latter  nerve  has  already  been  described,  and  the  former  should  be 
preserved  to  be  followed  in  the  dissection  of  the  front  of  the  leg. 

Directions. — The  outer  head  of  the  gastrocnemius  and  the  perforatus 
should  now  be  detached  close  to  their  origin,  and  turned  downwards. 
This  will  expose  the  whole  of  the  popliteus,  which  must  be  dissected 
carefully  from  the  jDOsterior  ligament  of  the  stifle  and  from  the  tibia,  in 
order  to  follow  the  popliteal  artery. 

The  Popliteal  Artery  (Plate  17)  is  the  direct  continuation  of  the 
femoral.  In  veterinary  anatomy  the  arbitrary  line  of  distinction  is 
usually  drawn  at  the  point  where  the  vessel  passes  in  between  the  heads 
of  the  gastrocnemius.  It  passes  over  the  posterior  ligament  of  the 
stifle,  where  it  is  covered  by  the  popliteus ;  and  at  the  tibio-fibular  arch 
it  bifurcates  to  form  the  anterior  and  posterior  tibial  arteries.  It  gives 
ofi" — (1)  articular  branches  to  the  stifle,  and  (2)  muscular  branches  to 
the  superficial  muscles  at  the  back  of  the  leg. 

The  Posterior  Tibial  Artery  (Plate  17)  is  much  the  smaller  of  the 
two  terminal  branches  of  the  popliteal.  In  the  first  part  of  its  course  it 
is  deeply  placed  beneath  the  popliteus  and  the  deep  and  accessory  flexors. 
As  it  descends,  it  becomes  more  superficial,  and  appears  at  the  posterior 
border  of  the  flexor  accessorius,  whose  tendon  it  follows  in  the  same  posi- 
tion, A  little  above  the  hock  it  forms  an  S-shapod  curve  that  brings  it 
into  company  with  the  terminal  part  of  the  posterior  tibial  nerve  ;  and 
passing  with  that  nerve  into  the  tarsal  sheath,  it  divides  at  the  back  of 
the  hock  into  the  two  plantar  arteries.  The  collateral  branches  of  the 
posterior  tibial  are — (1)  muscular  branches  to  the  deep  muscles  at  the 


DISSECTION    OF    THE    POSTERIOR    LIMB.  75 

back  of  the  leg ;  (2)  the  mitrient  artery  of  the  tibia  ;  (3)  a  retrograde 
branch  which,  emanating  from  the  second  curve  of  the  sigmoid  flexure, 
ascends  in  front  of  the  tendo  Achillis  ;  (4)  articidar  branches  to  the  tarsus. 

Veins.  The  foregoing  arteries  run  in  company  with  satellite  veins 
bearing  the  same  names. 

Directions. — The  front  of  the  leg  mvist  now  be  dissected ;  and  as 
the  first  step,  the  muscles  of  the  region  should  be  cleaned  and  isolated. 
These  are — the  extensor  pedis,  the  flexor  metatarsi,  and  the  peroneus. 
The  first  of  these  is  superposed  to  the  second  on  the  front  of  the  leg, 
while  to  the  outer  side  of  both  is  the  smaller  peroneus.  In  dissecting 
the  tendons  of  these  muscles  in  the  region  of  the  hock,  care  should 
be  taken  of  three  transverse  fibrous  bands  that  retain  the  tendons  in  posi- 
tion (Plates  18  and  19).  The  first  of  these  bands  is  fixed  by  its  extremities 
to  the  lower  end  of  the  tibia,  and  beneath  it  pass  the  tendons  of  the  ex- 
tensor pedis  and  flexor  metatarsi.  The  second  is  fixed  outwardly  to  os 
calcis ;  and,  passing  over  the  extensor  pedis  tendon,  it  is  attached  to  the 
flexor  metatarsi.  The  third  retains  the  tendons  of  the  extensor  pedis 
and  peroneus  in  position  at  the  upper  end  of  the  large  metatarsal  bone, 
to  which  its  extremities  are  attached. 

The  Extensor  Pedis  (Plate  18).  This  muscle  arises,  in  common  with 
the  tendinous  portion  of  the  flexor  metatarsi  (fig.  3,  page  76),  from  the  pit 
between  the  trochlea  and  external  condyle  of  the  femur.  It  has  a  thick, 
fusiform  mviscular  belly,  which  at  the  lower  third  of  the  leg  is  succeeded 
by  a  strong  tendon.  This  passes  over  the  front  of  the  hock,  and  under 
the  three  annular  bands  just  described.  It  then  descends  over  the  front 
of  the  metatarsus,  where  it  receives  the  insertion  of  the  short  extensor 
of  the  digit,  and  is  joined  by  the  tendon  of  the  peroneus.  In  the  dissec- 
tion of  the  digit  it  Avill  be  pursued  to  its  insertion  into  the  pyramidal 
eminence  of  the  os  pedis. 

Action. —  It  extends  in  succession  from  below  upwards  the  interphalan- 
geal  joints  and  the  fetlock,  and  finally  flexes  the  hock. 

The  Peroneus  (Plate  18).  This  is  a  much  smaller  muscle  than  the 
preceding,  to  whose  outer  side  it  lies.  Its  muscular  fibres  have  a  penni- 
form  arrangement,  and  arise  from  the  external  lateral  ligament  of  the 
stifle,  from  the  fibula,  and  from  the  aponeurotic  septum  between  it  and 
the  deep  flexor  of  the  phalanges.  Its  tendon  passes  through  the  groove 
on  the  external  tuberosity  (external  malleolus)  at  the  lower  end  of  the 
tibia,  and  then  over  the  outer  side  of  the  hock,  where  it  plays  in  a 
synovial  canal  formed  in  the  external  lateral  ligament.  Below  the 
hock  it  is  directed  obliquely  forward,  and  joins  the  tendon  of  the  extensor 
pedis  about  the  middle  of  the  metatarsus. 

Action. — The  same  as  the  preceding  muscle. 

Directions. — Cut  the  extensor  pedis  about  tlie  middle  of  the  leg,  and 
reflect  it  upwards  and  downwards  to  expose  the  next  muscle. 


70 


THE  ANATOMY  OF  THE  HORSE. 


The  Flexou  Metatarsi  (fig.  3).  Tliis  muscle  consists  of  two  parallel 
portions — a  superficial  and  a  deep.  The  superficial  division  exists  in 
the  fonn  of  a  tendinous  cord  with  little  or  no 
muscular  tissue,  and  arises,  in  common  with  the 
extensor  pedis,  from  the  pit  between  the  trochlea 
and  external  condyle  of  the  fenmr.  This  tendon 
of  origin  passes  through  the  notch  between  the 
anterior  and  external  tuberosities  at  the  upper 
end  of  the  tibia,  and  is  there  enveloped  by  the 
-B  synovial  membrane  of  the  femoro-tibial  joint.  In 
the  leg  the  tendinous  cord  rests  on  the  deep 
division  of  the  muscle,  and  passes  under  the 
annular  band  at  the  lower  extremity  of  the  tibia,  in 
company  with  the  tendon  of  the  extensor  pedis. 
At  the  front  of  the  hock  it  is  perforated  by  the 
tendon  of  the  deep  division,  and  then  bifurcates, 
one  branching  continuing  do^\'nwards  to  be  inserted 
into  the  upper  extremity  of  the  large  metatarsal 
bone,  the  other  deviating  outwards  to  be  inserted 
into  the  cuboid. 

The  deejy  division  of  the  flexor  metatarsi  rests  on 
the  tibia,  and  its  muscular  fibres  arise  from  the 
upper  part  of  the  outer  surface  of  that  bone.  At 
the  lower  end  of  the  tibia  it  is  succeeded  by  a 
tendon  which  perforates  that  of  the  superficial 
division  of  the   muscle,   and    divides,    one   branch 

Flexor  Mktat.rsi  Muscle  P'^^^"^8^   ^^    ^^    tWr<^(^   iutO  the    head    of   the    large 

(Chauieau).  metatarsal  bone,  along  wuth  the   large  division  of 

1.  Superficial  division  of  ^j^^  superficial  cord,  while  the  other  branch  is 
carried  inwards  to  bo  inserted  into  the  cuneiform 
parvum. 

Action. — To  flex  the  hock.  In  this  action  the 
superficial  tendinous  cord  plays  merely  a  mechanical 
part,  flexing  the  liock  when  the  stifle  is  flexed. 

The   External    Popliteal    Nerve    (Plate   18). 

This  nerve  has  already  been  seen  in  the  hip  and 

thigh.     It  is  a  branch  given  off  by  the  great  sciatic; 

and,    descending  in  front  of   the  parent  nerve,  it 

passes  between  the  biceps  and  the  outer  head  of  the  gastrocnemius, 

where,   a  little   behind  the  external  lateral  ligament   of  the  stifle,   it 

divides  into  the  musculo-cutaneous  and  anterior  tibial  nerves. 

The  MuscuLO-CuTANEOus  Nerve  descends  along  the  line  of  contact 
of  the  extensor  pedis  and  peroneus,  supplying  filaments  to  the  latter 
muscle.     At  the  lower  tliird  of  the  leg,  as  has  already  been  seen,  the 


Fig.  3. 


the  muscle;  2.  Its  origin 
from  the  femur ;  3.  Its 
cuboid  branch ;  4.  Its 
metatarsal  branch  ;  5. 
Deep  division  of  the 
muscle ;  ti.  Its  tendon 
passing  through  that  of 
the  sui)ei-ficial  division  ; 
7.  Cuneiform  branch  of 
this  tendon  ;  8.  Metatarsal 
branch  of  the  same ;  \K 
Extensor  pedis ;  A.  Pero- 
neus ;  B.  Insertion  of 
middle  straight  patellar 
ligament ;  C.  Femoral 
trochlea. 


DISSECTION    OF    THE    POSTERIOR    LIMB.  77 

cutaneous  division  of  the  nerve  pierces  the  deep  fascia,  and  passes  over 
the  hock  to  supply  the  skin  on  the  outer  side  of  the  metatarsus. 

The  Anterior  Tibial  Nerve  separates  from  the  preceding  at  an 
acute  angle,  and  a  few  inches  below  the  stifle  it  passes  under  cover  of 
the  extensor  pedis.  It  supplies  twigs  to  the  last-named  muscle,  the 
flexor  metatarsi,  and  the  short  extensor ;  and  descends  at  the  outer  side 
of  the  tibial  vessels,  afterwards  accompanying  the  large  metatarsal  artery 
to  terminate  in  the  skin  on  the  outer  side  of  the  digit. 

The  Anterior  Tibial  Artery  (Plate  19).  This,  it  will  be  recol- 
lected, is  one  of  the  terminal  branches  of  the  popliteal  artery.  Origi- 
nating behind  the  upper  extremity  of  the  tibia,  it  is  here  seen  coming- 
forwards  through  the  tibio-ftbular  arch.  It  descends  on  the  tibia, 
under  cover  of  the  flexor  metatarsi,  and  accompanied  by  the  nerve  and 
vein  of  the  same  name.  Gaining  the  front  of  the  hock,  it  rests  on  the 
anterior  tibio-tarsal  ligament,  covered  by  the  flexor  metatarsi  and 
extensor  pedis  at  their  line  of  contact.  Here  it  deviates  outwards  under 
the  tendon  of  the  last-mentioned  muscle,  and  divides  into  two  vessels 
of  unequal  size.  The  larger  of  these,  which  continues  the  direction 
of  the  parent  vessel,  is  the  large  metatarsal  artery ;  the  other  is  the 
perforating  metatarsal  artery ;  and  both  will  be  dissected  with  the 
metatarsus.  The  anterior  tibial  artery  gives  off  numerous  un-named 
muscular  branches  to  the  extensor  pedis,  flexor  metatarsi,  and  peroneus ; 
and  articular  branches  to  the  hock. 

The  Anterior  Tibial  Vein,  which  may  be  double,  keeps  close  com- 
pany with  the  artery.  It  is  formed  at  the  front  of  the  hock  by 
the  fusion  of  several  rootlets.  The  largest  of  these  is  the  upward 
continuation  of  the  deep  metatarsal  vein,  which  comes  forwards  through 
the  vascular  ca  nal  between  the  tarsal  bones.  After  passing  backwards 
through  the  tibio-fibular  arch,  the  anterior  joins  the  posterior  tibial 
vein  to  form  the  popliteal. 

the  metatarsus  and  digit. 

The  distal  portion  of  the  horse's  hind  limb,  beyond  the  lower  extremity 
of  the  tibia,  is  technically  termed  the^:)es,  as  it  corresponds  to  the  foot 
of  man.  The  tarsus,  or  hock,  represents  the  human  ankle;  the  part 
between  the  tarsus  and  fetlock  corresponds  to  the  body  of  the  human 
foot,  and  is  termed  the  metatarsus ;  while  the  rest  of  the  limb,  beyond 
the  fetlock,  is  the  digit,  and  is  the  homologue  of  man's  third  toe. 

Surface-marking. — Extending  down  the  middle  line  in  front  is  the 
tendon  of  the  extensor  pedis,  which,  a  little  below  the  tarsus,  is  joined 
obliquely  by  the  tendon  of  the  peroneus.  Behind  the  metatarsus,  and 
resting  on  the  bone,  is  the  suspensory  ligament  ;  and  behind  that 
again  are  the  deep  and  superficial  flexors  of  the  foot.  The  edges  of 
these  structures  can  be  distinctly  seen  or  felt  in  the  living  animal,  and 
jn  the  dead  subject  they  may  be  identified  by  a  reference  to  Plate  19. 


78  THE  ANATOMY  OF  THE  HORSE. 

At  the  upper  purt  of  the  inner  face  of  the  metatarsus  is  a  flattened 
horny  callosity,  or  chestnut;  and  another  horny  excrescence,  in  the 
form  of  a  spur,  or  err/ot,  is  concealed  in  the  tuft  of  hair  behind  the 
fetlock.  By  manipulation  in  the  neighlwurhood  of  the  heels,  the  lateral 
cartilages  may  be  felt. 

Directions. — Remove  the  entire  remaining  portion  of  skin  from  the 
limb  ;  and  if  it  is  intended  to  study  on  the  same  preparation  the  parts 
contained  within  the  hoof,  this  must,  before  the  removal  of  the  skin,  be 
detached  by  force  in  the  manner  described  at  page  35.  The  various 
structures  are  now  to  be  defined  by  dissection,  in  the  order  of  the 
following  description ;  and  while  the  vessels  and  nerves  are  being 
cleaned,  care  umst  l)e  taken  of  the  small  lumbricales  muscles,  which  lie 
on  the  tendon  of  the  deep  flexor,  above  the  fetlock. 

Cutaneous  Nerves. — Descending  over  the  inner  side  of  the  hock  and 
metatarsus  arc  twigs  of  the  internal  saphenous  and  posterior  tibial 
nerves,  and  on  the  outer  side  of  the  same  regions  are  branches  of  the 
external  saphenous  and  musculo-cutaneous  nerves. 

The  Large  INIetatarsal  Artery  {Dorsalis  2)edis  of  man)  (Plate  19) 
is  the  larger  branch  resulting  from  the  division  of  the  anterior  tibial 
artery  at  the  front  of  the  tarsus.  It  inclines  outwai-ds  and  down- 
wai-ds  under  the  extensor  brevis  and  the  peroneus,  and  places  itself  in 
the  groove  formed  on  the  outer  side  of  the  metatarsus  by  the  junction  of 
the  large  and  outer  small  metatarsal  bones.  Along  this  groove  it 
descends  in  company  with  the  slender  continuation  of  the  anterior 
tibial  nerve,  until,  a  little  above  the  button  of  the  smaller  bone,  it 
passes  to  the  back  of  the  metatarsus  by  penetrating  between  the  two 
bones.  Finally,  it  bifurcates  above  the  fetlock,  between  the  two 
divisions  of  the  suspensory  ligament,  to  form  the  digital  arteries.  It 
gives  ofl"  numerous  un -named  twigs  to  the  skin,  tendons,  etc. 

Descending  in  the  metatarsal  region,  there  are  other  four  arteries 
besides  the  vessel  just  described.  They  will  be  found,  one  at  each  side 
of  the  flexor  tendons,  in  com2)any  with  the  vein  and  nerve,  and  another 
at  each  edge  of  the  suspensory  ligament,  within  the  splint  bone  of  the 
same  side.  All  of  these  are  branches  of  an  arterial  arch  formed  across 
the  origin  of  the  suspensory  ligament  from  the  back  of  the  tarsus.  The 
arch  corresponds  to  the  subcarpal  arch  of  the  anterior  limb,  and  is 
formed  as  follows  : — 

The  Perforating  Metatarsal  Artery,  the  smaller  branch  resulting 
from  the  division  of  the  anterior  tibial  artery,  passes  from  the  front  to 
the  back  of  the  tarsus  by  the  canal  between  the  cuboid,  scaphoid,  and 
cuneiform  bones.  Here  it  unites  with  the  outer  and  inner  plantar 
divisions  of  the  posterior  tibial,  which  descend  in  the  tarsal  sheath,  one 
on  each  side  of  the  perforans  tendon.  Of  the  four  vessels  that  spring 
from  the  arch  thus  formed,   the  two  that   descend  with  the  plantar 


DISSECTIOX    OF    THE    POSTERIOR    LIMB.  79 

nerves  at  the  side  of  the  flexor  tendon  are  nn-named  and  slender  (Plate 
19).  The  other  two  are  termed  the  plantar  interosseous  metatarsal 
arteries.  This  may  be  regarded  as  the  most  typical  arrangement  of  the 
arteries  here,  but  it  is  not  constant.  Sometimes  the  inner  plantar 
artery  is  directly  continued  as  the  satellite  vessel  of  the  internal  plantar 
nerve  in  the  metatarsus,  the  outer  plantar  artery  alone  uniting  with  the 
perforating  metatarsal  artery. 

The  External  Plantar  Interosseous  Artery  is  very  slender.  It 
descends,  as  beforesaid,  between  the  outer  sj^lint  bone  and  the  edge  of 
the  suspensory  ligament ;  and  above  the  fetlock  it  anastomoses  with  a 
recurrent  twig  from  the  large  metatarsal  artery. 

The  Internal  Plantar  Interosseous  Artery,  a  vessel  of  consider- 
able size,  descends  between  the  inner  splint  bone  and  the  edge  of  the 
suspensory  ligament.  Above  the  lower  extremity  of  that  bone  it  inclines 
towards  the  middle  of  the  limb  to  join  the  large  metatarsal  artery.  It 
svipplies  the  nutrient  artery  of  the  large  metatarsal  bone. 

The  Digital  Arteries  (Plate  19).  These  arteries  separate  at  an 
acute  angle,  in  passing  backwards  between  the  branches  of  bifurcation 
of  the  suspensory  ligament.  For  the  remainder  of  their  course  they 
are  identical  with  the  homonymous  vessels  of  the  fore  limb.  For  their 
description,  turn  to  page  28. 

The  Digital  Veins  (Plate  19).  These  are  the  satellites  of  the  digital 
arteries,  in  front  of  which  they  ascend.  They  drain  away  the  blood 
from  the  venous  plexuses  within  the  hoof;  and,  uniting  with  one 
another  above  the  fetlock,  they  form  an  arch  between  the  deep  flexor  and 
the  suspensory  ligament.      From  this  arch  spring  the  metatarsal  veins. 

The  Metatarsal  Veins  are  three  in  number : — 

1.  The  Internal  Metatarsal  Vein  ascends  in  front  of  the  inner  edo-e  of 
the  deep  flexor  tendon,  in  company  -with  the  internal  plantar  nerve  and 
a  slender  artery.  The  vein  is  the  most  anterior  of  the  three  structures 
and  the  slender  artery  is  between  the  vein  and  the  nerve.  At  the 
upper  third  of  the  metatarsus  the  vein  deviates  forwards,  crossino-  the 
inner  splint  bone  and  the  large  metatarsal  obliquely,  to  gain  the  inner 
side  of  the  hock,  above  which  it  is  continued  as  the  anterior  root  of  the 
internal  saphena  vein.  The  coui'se  of  the  vein  over  the  hock  is  generally 
apparent  in  the  living  animal,  and  when  very  prominent  it  constitutes 
the  so-called  "blood-spavin." 

2.  The  Extenml  Metatarsal  Vein  (Plate  19)  ascends  on  the  inner  edo-e 
of  the  deep  flexor,  having  the  same  relationship  to  nerve  and  artery  as 
the  internal  vein.  After  communicating  with  the  deep  vein,  it  is  con- 
tinued through  the  tarsal  sheath  to  become  the  posterior  root  of  the 
internal  saphena. 

3.  The  Deep  Ifetatarsal  Vein  ascends  between  the  suspensory  liga- 
ment and  the  large  metatarsal  bone  ;  and  passing  from  the  back  to  the 


80  THE  ANATOMY  OF  THE  HORSE. 

front  of  the  liock,  ])y  the  vascular  canal  for  the  perforating  metatarsal 
artery,  it  is  continued  as  the  anterior  tibial  vein. 

The  Plantar  Nerves.  These  nerves  rcsiilt  from  the  bifurcation  of 
the  posterior  tibial  nerve  when  it  gains  the  back  of  the  tarsus.  They 
accompany  the  perforans  tendon  in  the  tarsal  sheath  ;  and  diverging 
from  one  another,  they  descend  in  the  metatarsal  region,  one  at  each 
side  of  the  deep  flexor  tendon.  Each  is  accompanied  in  the  mctatai'sus 
by  the  metatarsal  vein  of  that  side,  and  by  a  slender  artery  from  the 
vascular  arch  at  the  back  of  the  tarsus.  A  little  below  the  middle  of 
the  metatarsus  the  inner  nerve  detaches  a  considerable  branch  that 
winds  obliquely  downwards  and  outwards  behind  the  flexor  tendons  to 
join  the  outer  nerve  above  the  level  of  the  button  of  the  splint  bone. 
At  the  fetlock  each  nerve,  coming  into  relation  with  the  digital  vessels, 
resolves  itself  into  three  branches  for  the  supply  of  the  digit.  These 
are  identical  in  their  arrangement  with  the  like  branches  of  the  plantar 
nerves  in  the  fore  limb,  for  the  description  of  which,  turn  to  page  30. 

The  student  must  now  pursue  the  dissection  of  the  following  muscles 
which  have  already  been  dissected  in  the  leg,  viz.,  the  extensor  pedis 
and  peroneus  on  the  front  of  the  limb,  and  the  superficial  and  deep 
flexors  behind.  In  addition  to  these,  there  are  the  short  extensor  of 
the  foot,  the  lumbricales,  and  the  interossei,  which  entirely  belong  to 
this  region ;  and  since  they  are  of  small  size,  and  might  easily  be  over- 
looked or  iujui-ed,  their  dissection  must  be  first  undertaken. 

The  Lumbricales  (Plate  19)  and  Interossei  Muscles.  These  exactly 
resemble  the  muscles  of  the  same  name  in  the  anterior  member.  Turn, 
therefore,  to  the  description  of  the  latter  given  at  page  31,  substituting 
the  vf ord  foot  for  hand,  toes  ioY  finf/ers,  and  metatarsal  for  metacarpal. 

The  Short  Extensor  of  the  foot  (extensor  brevis  digitomm  of  man) 
(Plate  19).  Look  for  this  small  muscle  at  the  front  of  the  tarsus,  in  the 
angle  of  union  of  the  extensor  pedis  and  peroneus  tendons.  It  arises 
from  the  os  calcis  and  astragalus,  and  is  inserted  into  the  united  tendon 
of  the  above-mentioned  muscles,  to  whose  action  it  is  auxiliary. 

The  Extensor  Pedis  tendon  (Plate  19)  descends  along  the  middle 
line  of  the  limb  in  front,  to  be  inserted  into  the  pyramidal  eminence  of 
the  OS  pedis.  Above  the  middle  of  the  metatarsus  it  receives  on  its 
outer  side  the  tendon  of  the  peroneus,  and  at  the  same  point  it  is  joined 
by  the  short  extensor.  A  small  synovial  bursa  is  interposed  between 
the  tendon  and  the  anterior  ligament  of  the  fetlock,  but  at  the  front  of 
the  intcrphalangeal  joints  the  ligament  supports  directly  the  articular 
synovial  membranes.  At  the  middle  of  the  first  phalanx  the  tendon 
is  joined  on  each  side  by  a  strong  band  that  descends  from  the 
suspensory  ligament. 

Action. — It  extends  in  succession  from  below  upwards  the  intcrphal- 
angeal joints  and  the  fetlock,  and  finally  it  flexes  the  hock. 


DISSECTION    OF    THE    POSTERIOR    LIMB.  81 

The  Peroneus  (Plate  19).  The  tendon  of  this  muscle  emerges  from  the 
thecal  canal  in  the  external  lateral  ligament  of  the  tarsus,  and  joins  the 
tendon  of  the  last-described  muscle  about  the  middle  of  the  metatarsus. 

Action. — The  same  as  the  preceding  muscle. 

The  Superficial  Flexor  (flexor  pedis  perforatus)  (Plate  19).  The 
tendon  of  this  muscle,  after  playing  over  the  os  calcis,  descends  on  the 
middle  line  of  the  limb  to  the  back  of  the  fetlock,  where  it  forms  a 
remarkable  ring  for  the  passage  of  the  tendon  of  the  deep  flexor.  Be- 
yond this  point  the  tendon  bifurcates,  and  each  half  is  inserted  into  the 
upper  extremity  of  the  second  phalanx,  on  its  lateral  aspect.  In  con- 
nection with  the  tendon  of  this  and  the  next  muscle  there  is  developed  an 
extensive  synovial  apparatus,  termed  the  metatarso-phalangeal  sheath, 
which  exactly  resembles  the  metacarpo-phalangeal  sheath  of  the  fore 
limb,  described  at  page  34. 

Action. — It  flexes  successively  the  pastern  and  fetlock  joints ;  and,  by 
its  insertion  into  the  os  calcis,  it  is  also  an  extensor  of  the  hock-joint. 
It  also  mechanically  maintains  the  hock  in  a  state  of  extension  as  long 
as  the  hip  and  stifle  joints  are  kept  extended  by  their  proper  muscles. 

The  Deep  Flexor  (flexor  perforans)  (Plate  19).  The  tendon  of 
this  muscle,  after  its  passage  through  the  tarsal  sheath,  descends 
between  the  suspensory  ligament  and  the  superficial  flexor.  At  the 
fetlock  it  passes  through  the  ring  of  the  last-named  muscle,  descends 
behind  the  digit,  plays  over  the  navicular  bone,  and  finally  becomes 
inserted  into  the  solar  surface  of  the  os  pedis  (see  page  42).  At  the 
upper  part  of  the  metatarsus  it  receives  the  check  band,  or  subtarsal 
ligament,  which  is  analagous  to  the  subcarpal  ligament  of  the  fore  limb, 
but  not  so  strong.  Like  the  analagous  structure  in  the  fore  limb,  it  is 
involved  in  sprain  of  the  back  tendons.  A  little  lower  the  deep  flexor  is 
joined  on  its  outer  side  by  the  tendon  of  the  flexor  accessorius. 

Action. — It  flexes  successively  from  below  upwards  the  interphalangeal 
joints  and  the  fetlock. 

Directions. — For  the  description  of  the  foot,  which  is  identical  in  the 
fore  and  hind  limbs,  turn  to  page  35.  If  the  student  has  already  dis- 
sected the  foot  in  a  fore  limb,  he  may  proceed  at  once  to  the  articulations. 

THE  stifle-joint  (pLATES    17    AND    18). 

This  corresponds  to  the  knee-joint  of  man.  It  comprises — (1)  the  articu- 
lation between  the  patella  and  the  femoral  trochlea;  and  (2)  the  articiila- 
tion  between  the  condyles  of  the  femur  and  the  proximal  end  of  the  tibia. 

Directions. — The  various  structures  in  connection  with  the  joint  are 
to  be  examined  in  the  order  of  the  following  description  ;  and  in  order 
to  expose  them,  the  muscles,  fat,  etc.,  are  to  be  removed  from  around 
the  joint,  care  being  taken,  in  the  first  stage  of  the  dissection,  to  pre- 
serve the  thin  femoro-patellar  capsule  intact. 

G 


82  THE   AXATOMY    OF   THE    HORSE. 

FEMORO-PATELLAR  ARTICULATION. 

Movements. — This  joint  is  commonly  classified  as  an  arthroJia.  The 
movements  (see  page  43)  of  the  patella  on  the  trochlea,  however,  are  not 
those  of  simple  gliding,  but  of  glidinrj  with  coaptation.  In  the  latter 
movement,  while  the  patella  moves  as  a  whole  upwards  or  downwards, 
successive  areas  of  its  articular  surface  come  into  contact  with  the 
trochlea.  These  movements  take  place  at  the  same  time  as  the  move- 
ments in  the  femoro-tibial  articulation.  In  complete  extension  of  that 
joint  the  patella  lies  at  the  upper  part  of  the  trochlea,  and  the  three 
straight  patellar  ligaments  are  tense.  When  flexion  takes  place,  these 
ligaments  become  relaxed,  and  the  patella  descends  over  the  trochlea 
till  it  rests  at  its  lower  part. 

The  ligaments  of  the  joint  are — one  capsular,  two  lateral,  and  three 
straight. 

The  Capsular  Ligament  is  loose  and  membranous,  and  it  supports 
the  synovial  membrane.  It  is  attached,  on  the  one  hand,  to  the  margin 
of  the  patellar  articular  surface,  and,  on  the  other,  at  the  periphery  of 
the  trochlea. 

The  Lateral  Ligaments  are  two  thin,  riband-shaped  bands,  stretching, 
one  on  each  side  of  the  joint,  from  the  femur  to  the  patella.  They  serve 
to  sti-eugthen  the  capsular  ligament,  from  which  they  are  not  distinct. 

The  Straight  Patellar  Ligaments.  These  correspond  to  the  single 
ligamentum  jicitellce  of  the  human  knee.  They  are  three  in  number,  and 
are  disfiitgmshed  as  external,  middle,  and  internal.  All  three  ligaments 
are  attached  superiorly  to  the  anterior  surface  of  the  patella,  the  inner 
one  having  a  fibro-cartilaginous  thickening  which  extends  the  articular 
surface  of  the  patella,  and  glides  on  the  inner  ridge  of  the  femoral 
trochlea.  The  middle  ligament  lies  on  a  deeper  plane  than  the  other 
two,  and  rests  inferiorly  in  the  vertical  groove  on  the  anterior  tuberosity 
of  the  tibia.  It  is  inserted  into  the  lower  part  of  this  groove,  and  a 
small  synovial  bursa  is  developed  between  the  ligament  and  the  bone 
above  the  point  of  insertion.  The  external  and  internal  ligaments  are 
inserted  into  the  same  tuberosity,  one  on  each  side  of  the  attachment  of 
the  middle  ligament.  These  three  ligaments  may  be  regarded  as  the 
terminal  tendon  of  the  quadriceps  extensor  cruris,  whose  action  they 
transmit  tc  the  bones  of  the  leg. 

Synovial  Membrane.  This  will  be  exposed  by  incising  the  capsular 
ligament.  It  lines  the  inner  surface  of  that  ligament,  and  extends 
upwards  beyond  the  trochlea,  forming  a  protrusion  under  the  quad- 
riceps extensor  cruris.  Inferiorly  it  is  in  contact  with  the  synovial 
membranes  of  the  femoro-tibial  joint,  and  sometimes  it  communicates 
with  them. 

It  is  a  point  worthy  of  notice  in  connection  with  the  interior  of  the 
joint,  that  the  inner  ridge  of  the  femoral  trochlea  is  much  higher  than 


DISSECTION    OF    THE    POSTERIOR   LIMB. 


83 


the  outer ;  and  when  the  patella  is'  dislocated,  it  is  carried  outwards 
over  the  external  ridge. 

THE  FEMORO-TIBIAL  ARTICULATION  (piG.   4). 

Movements. — This  is  a  ginglymus,  or  hinge  joint,  in  which  the  move- 
ments are  principally  ^ea;ion.  and  extension.  In  extension  the  bones  of 
the  leg  are  carried  forwards,  but  cannot  be  brought  into  a  straight  line 


Fig.  4. 

A.  Femoro-tibial  Ligaments,  back  view. 

1.  External  lateral  ligament ;  2.  Internal  lateral  ligament ;  3.  Inner  semilunar  fibro-cartilage  ; 

4.  Outer  semilunar  fibro-cartilage,  with  5,  and  6,  the  femoral  and  tibial  attachments  (coronary 
ligaments)  of  its  posterior  extremity ;  7.  Posterior  crucial  ligament ;  8.  Anterior  crucial  ligament  ; 
9.  Head  of  fibula. 

B.  Antero-posterior  Vertical  Section  of  the  Femoro-tibial  Articulation  to  show 

THE  Crucial  Ligaments. 

1.  The  posterior  crucial  ligament ;  2.  Anterior  crucial  ligament ;  3.  External  lateral  ligament ;  4, 

5,  and  6,  as  in  A. 

with  the  femur,  the  movement  being  arrested  by  tension  of  the  lateral 
ligaments  and  of  the  anterior  crucial  ligament.  The  contrary  movement, 
flexion,  is  finally  arrested  by  tension  of  the  posterior  crucial  ligament. 
A  slight  degree  of  lateral  movement  and  rotation  can  be  produced  when 
the  joint  is  flexed. 

Lateral  Ligaments. — These  are  two  strong  fibrous  cords,  placed  one 
on  each  side  of  the  joint.    The  external  is  fixed  above  to  the  higher  of  the 


84  THE  ANATOMY  OF  THE  HORSE. 

two  pits  on  the  external  condyle  of  the  femur,  where  it  covers  the  origin 
of  the  popliteus  from  the  lower  pit.  It  descends  over  the  external 
tuberosity  of  the  tibia,  a  synovial  bm-sa  being  interposed,  and  is  inserted 
into  the  head  of  the  fibula.  The  internal  is  longer,  but  more  slender, 
than  the  preceding.  It  is  fixed  above  to  a  small  tiibercle  on  the  inner 
condyle,  plays  over  the  inner  edge  of  the  tibial  articular  surface,  and  is 
inserted  into  the  internal  tuberosity  of  the  tibia. 

The  Posterior  Ligament  is  of  a  flattened,  membranous  character,  and 
consists  of  a  superficial  and  a  deep  layer,  which  are  separable  from  each 
other  superiorly,  but  blended  below.  Superiorly  the  ligament  is  attached 
to  the  posterior  surface  of  the  femur  above  the  condyles ;  below  it  is 
inserted  into  the  corresponding  surface  of  the  tibia,  just  below  the 
margin  of  the  articular  surface ;  while  laterally  its  margins  blend  with 
the  lateral  ligaments.  The  superficial  surface  of  the  ligament  is  related 
to  the  popliteal  vessels,  and  to  the  gastrocnemius,  flexor  perforatus,  and 
popliteus  muscles.  Its  deep  face  serves  to  support  the  synovial  mem- 
branes of  the  joint,  and  is  partly  adherent  to  the  semilunar  cartilages 
and  posterior  crucial  ligament.  The  ligament  presents  apertures  for 
the  transmission  of  vessels  to  the  interior  of  the  joint. 

Synovial  Membranes.  These  are  two  in  iiumber,  one  for  each  condyle 
of  the  femur  and  corresponding  part  of  the  ai-ticular  surface  of  the  tibia. 
They  are  separated  from  each  other  by  the  crucial  ligaments  in  the 
interior  of  the  joint;  while  behind,  and  at  the  sides,  they  line  the 
posterior  and  lateral  ligaments.  In  front  they  are  in  contact  with  the 
synovial  capsule  of  the  femoro-patellar  articulation,  and  are  supported 
by  a  pad  of  fat,  which  separates  them  from  the  straight  ligaments  of 
the  patella.  A  communication  frequently  exists  in  front  between  these 
synovial  capsules  and  that  for  the  gliding  of  the  patella.  These 
synovial  membranes  invest  the  semilunar  cartilages ;  and  the  external 
one  covers,  in  addition,  the  tendon  of  origin  of  the  popliteus,  and  the 
common  tendon  of  origin  of  the  flexor  metatarsi  and  extensor  of  the 
digit. 

Directions. — The  posterior  ligament  sliould  now  be  cut  away;  and 
the  patella  being  thrown  down,  the  synovial  membrane  and  fat  should 
be  removed  from  the  fx-ont  of  the  joint.  The  joint  should  then  be 
strongly  flexed,  in  order  to  expose,  as  far  as  possible,  the  crucial  liga- 
ments in  the  intercondyloid  groove.  The  rims  of  the  semilunar 
cartilages   and   their   coronary   ligaments   will   at   the   same   time   be 


The  Crucial  Ligaments  are  two  strong  fibrous  cords  stretching  between 
the  femur  and  the  tibia,  and  lodged  in  the  intercondyloid  groove.  They 
cross  one  another  somewhat  like  the  limbs  of  the  letter  X,  and  hence 
their  name.  They  are  distinguished  as  anterior  and  posterior.  The 
anterior^  the  most  external  of  the  two,  is  attached  superiorly  to  the 


DISSECTION    OF    THE    POSTERIOR    LIMB.  85 

intercoudyloid  groove,  and  to  the  external  condyle  of  the  femur  where 
it  bounds  that  groove.  Its  fibres  have  a  slightly  spiral  arrangement, 
and  extend  downwards  and  forwards  to  be  inserted  into  the  summit  of 
the  tibial  spine.  The  2^osterio7-  ligament  is  longer  than  the  anterior, 
and  is  fixed  superiorly  to  the  intercoudyloid  groove  and  inner  condyle. 
It  extends  downwards  and  backwards  to  be  fixed  to  a  special  tubercle 
on  the  back  of  the  tibia  below  the  rim  of  its  articular  surface.  These 
two  ligaments  bind  the  femoral  and  tibial  articular  surfaces  closely 
together,  and  at  the  same  time  restrict  the  movements  of  the  joint,  the 
anterior  ligament  being  put  upon  the  stretch  during  extension,  and  fin- 
ally arresting  that  movement,  while  the  posterior  ligament  plays  the 
same  part  with  regard  to  flexion. 

The  Inter-articdlar  or  Semilunar  Fibro-cartilages.  These  are  two 
crescentic  or  sickle-shaped  pieces  of  fibro-cartilage,  interposed  between 
the  condyles  of  the  femur  and  the  articular  surface  of  the  tibia.  The 
convex  margin  of  each  is  turned  outwards,  and  is  much  thicker  than  the 
concave  edge,  which  embraces  the  tibial  spine,  and  is  so  thin  as  to  be 
translucent.  The  lower  surface  of  each  is  flattened  to  rest  on  the  tibia, 
but  the  upper  surface  is  hollowed  to  embrace  the  femoral  condyle. 
They  are  fixed  in  position  as  follows : — The  anterior  extremity  of  the 
inner  fibro-cartilage  is  fixed  into  an  excavation  in  front  of  the  tibial 
spine,  while  its  posterior  end  is  similarly  fixed  behind  the  spine.  The 
outer  cartilage  is  fixed  by  its  anterior  extremity  in  front  of  the  spine, 
while  its  posterior  extremity  is  bifid,  having  an  upper  slip  inserted  into 
a  depression  at  the  posterior  part  of  the  intercoudyloid  groove,  and  a 
lower  into  the  rim  of  the  tibial  articular  surface,  partly  under  cover  of 
the  posterior  interosseous  ligament.  These  slips  of  insertion  at  the 
extremities  of  the  fibro-cartilages  are  sometimes  termed  the  coronary 
ligaments,  three  of  which  belong  to  the  outer,  and  two  to  the  inner, 
fibro-cartilage.  Although  these  insertions  serve  to  prevent  the  total 
displacement  of  the  fibro-cartilages,  some  degree  of  movement  is,  never- 
theless, permitted  to  the  latter ;  for  it  will  be  noticed,  that  during 
flexion  they  are,  as  it  were,  squeezed  towards  the  front  of  the  joint, 
while  in  extension  they  are  carried  backwards. 

Directions. — If  the  internal  lateral  ligament  be  now  cut,  and  the 
internal  condyle  removed  with  the  saw,  a  better  view  will  be  obtained  of 
the  crucial  ligaments ;  after  which,  complete  sepai'ation  of  the  femur  and 
tibia  should  be  effected  by  cutting  the  remaining  lateral  ligament,  the 
crucial  ligaments,  and  the  slip  of  insertion  of  the  external  fibro-cartilage 
at  the  back  of  the  intercoudyloid  groove.  This  will  expose  thoroughly 
the  semilunar  fibro-cartilages. 

TiBio-FiBULAR  ARTICULATION.  In  the  horse  the  amount  of  movement 
permitted  between  the  bones  of  the  leg  is  very  restricted,  and  not 
appreciable  on  the  general  movements  of  the  limb.     Where  the  head  of 


86  THE  AXATOMY  OF  THE  HORSE. 

the  fibula  is  opposed  to  the  rough  diarthrodial  facet  on  the  external 
tuberosity  at  the  upper  end  of  the  tibia,  short  and  strong  peripheral 
fibres  pass  between  the  two  bones,  and  bind  them  closely  together.  An 
interosseous  ynevibi'ane  extends  across  the  tibio-fibular  arch,  and  is  perfor- 
ated by  the  anterior  tibial  vessels.  Just  above  the  aperture  for  the 
transmission  of  these  vessels  the  fibres  of  the  ligament  are  disposed  in 
opposite  directions,  like  the  limbs  of  the  letter  X. 

Where  the  osseous  substance  of  the  fibula  ceases,  a  fibrous  cord  begins, 
and  this  is  carried  downwards  to  the  region  of  the  external  tuberosity 
at  the  lower  end  of  the  tibia,  where  it  mixes  its  fibres  with  the  external 
lateral  ligament  of  the  tibio-tarsal  joint. 

THE  TARSUS  (fIG.    5). 

Several  articulations  are  formed  in  the  tarsus,  or  hock ;  and  these 
are  of  very  unequal  importance  as  regards  the  amount  of  movement 
permitted.  The  most  important  of  them  is  that  corresponding  to  the 
ankle-joint  of  man,  which  is  formed  between  the  astragalus  and  the 
lower  extremity  of  the  tibia ;  and  attention  should  first  be  given  to  the 
movements  that  take  place  here.  This  is  one  of  the  most  typical 
ginglymoid  joints  in  the  body,  the  movements  being  limited  to  flexion 
and  extension.  It  will  be  observed  that  in  flexion  the  distal  part  of  the 
limb  does  not  move  in  the  plane  of  the  leg,  but  deviates  a  little  out- 
wards, and  that  in  extension  the  movement  is  arrested  by  tension  of 
the  lateral  ligaments  before  the  distal  portion  of  the  limb  is  brought 
into  the  same  straight  line  as  the  leg. 

In  the  other  articulations  found  in  connection  with  the  tarsus  the 
movements  are  of  a  very  restricted  character,  and  are  not  concerned  in 
the  general  movements  of  the  limb.  They,  however,  serve  a  no  less 
important  purpose  in  the  joint,  distributing  and  equalising  pressure, 
and  obviating  the  bad  effects  which  concussion  would  have  been  likely 
to  produce  in  the  tarsus,  had  it  been  one  rigid  structure. 

Directions. — The  ligaments  of  the  tarsus  are  both  numerous  and  com- 
plicated, and  the  best  order  of  their  dissection  is  that  in  which  they  are 
hereafter  described.  Since  one  set  of  ligaments  must  be  removed  in 
order  to  expose  the  following  set,  the  dissector  should  not  proceed  with 
undue  rapidity. 

TiBio-TARSAL  LiGAMENTS. — Thcsc  are  four  in  number,  viz.,  two  lateral, 
an  anterior,  and  a  posterior. 

The  External  Lateral  Ligameyit  consists  of  a  superficial  and  a  deep 
fasciculus,  which  cross  one  another  like  the  legs  of  the  letter  X.  The 
superficial  division,  which  is  the  larger  of  the  two,  is  fixed  superiorly 
to  the  posterior  part  of  the  external  tuberosity  at  the  lower  end 
of  the  tibia,  while  inferiorly  its  fibres  are  inserted  into  the  astrag- 
alus,   OS    calcis,    cuboid,    large    metatarsal   bone,    and    external    small 


DISSECTION    OP    THE    POSTERIOR    LIMB. 


87 


metatarsal.  It  is  perforated  by  the  thecal  canal  for  the  passage  of  the 
peroneus  tendon.  The  deep  division  of  the  ligament  extends  down- 
Avards  and  backwards  from  its  point  of  attachment  to  the  forepart  of 
the  external  tuberosity  of  the  tibia,  and  it  becomes  inserted  by  distinct 
slips  into  the  astragalus  and  os  calcis.  In  order  to  expose  it  thoroughly, 
the  superficial  division  should  be  cut  at  its  point  of  attachment  to  the 
tibial  tuberosity,  and  dissected  downwards,  the  difference  of  direction 
serving  to  distinguish  the  fibres  of  the  two  divisions. 

The  Tntet'nal  Lateral  Ligament  is,  like  the  preceding,  a  composite  liga- 
ment, and  consists  of  three  divisions,  which  may  be  distinguished  as 


A. — LlOAMENTS   OF   THE   TaRSUS,    FRONT    VIEW. 

1.  Superficial  fasciciilns  of  the  internal  lateral  ligament  (cut)  ;  2.  Middle  fasciculus  of  the  same 
(two  slips) ;  3.  Deep  fasciculus  of  the  same  ;  4.  Superficial  fasciciilus  of  the  external  lateral  liga- 
ment ;  5.  Deep  fasciculus  of  tlie  same  ;  6.  Astragalo-metatarsal  ligament ;  7.  Canal  for  tlie  perforat- 
ing metatarsal  artery ;  8.  Anterior  cuboido-cunean  ligament ;  9.  Anterior  cuboido-scaphoid  liga- 
ment ;  10.  Cuboid  insertion  of  the  fle.xor  metatarsi. 

B.— Ligaments  of  the  Tarsus,  back  view. 
1.  External    lateral    ligament ;    2.  Internal   lateral   ligament ;     3.  Tarso-metatarsal    ligament ; 
4.  Fibro-cartilaginous  thickening    of    the  posterior  ligament.      5.  Calcaneo-metatarsalCligament ; 
6.  Subtarsal  ligament,  or  check-band  to  perforans  tendon  ;  7.  Suspensory  ligament.  Hbr"'" 

superficial,  middle,  and  deep.  The  superficial  division,  the  largest  of 
the  three,  is  fixed,  on  the  one  hand,  to  the  internal  tuberosity  at  the 
lower  end  of  the  tibia,  and,  on  the  other,  to  the  astragalus,  scaphoid, 


88  THE  ANATOMY  OP  THE  HORSE. 

large  and  small  cuneiforms,  and  large  and  internal  small  metatarsal 
bones.  The  middle  division  is  of  intermediate  size ;  and  in  order  to 
expose  it,  the  superficial  division  must  be  cut,  and  dissected  downwards. 
Above  it  is  attached  to  the  internal  tuberosity  of  the  tibia ;  and,  passing 
downwards  and  backwards,  it  is  inserted  by  distinct  slips  into  the 
astragalus  and  os  calcis.  The  deep  division  is  very  slender,  and 
stretches  between  the  tibia  and  the  astragalus,  under  cover  of  the  middle 
fasciculus,  which  must  be  removed  in  order  to  expose  it. 

The  Antenor  Ligament  is  membranous  and  four-sided.  It  is  fixed 
above  to  the  tibia  ;  and  below  to  the  astragalus,  scaphoid,  cuneiform 
magnum,  and  astragalo-metatarsal  ligament  ;  while  on  each  side  it 
blends  with  the  lateral  ligament.  The  posterior  surface  of  the  ligament 
is  lined  by  the  synovial  membrane  of  the  joint.  The  anterior  surface 
is  related  to  the  anterior  tibial  vessels,  and  to  the  flexor  metatarsi 
and  extensor  pedis  tendons.  Towards  its  inner  side  the  ligament 
is  unsupported ;  and  hence,  when  the  synovial  membrane  becomes 
dropsical,  the  distension  shows  at  that  point,  constituting  a  "bog- 
spavin." 

The  Posiemor  Ligament  is  of  a  similar  form  to  the  preceding.  It  is 
fixed  above  to  the  tibia,  below  to  the  astragalus  and  os  calcis,  and  at 
the  sides  to  the  lateral  ligaments.  Its  anterior  surftxce  supports  the 
synovial  membrane  of  the  joint ;  Avhile  the  posterior  is  lined  by  the 
synovial  membrane  of  the  tarsal  sheath,  and  presents  a  fibro-cartila- 
ginous  thickening  where  the  perforans  tendon  plays  over  it.  This  tendon 
affords  support  to  the  posterior  ligament,  which  therefore  does  not 
bulge  so  readily  as  the  anterior  ligament ;  but  in  a  case  of  extreme 
distension  of  the  synovial  membrane,  the  swelling  shows  itself  at  the 
back  of  the  joint. 

The  Synovial  31emh-ane  is  supported  by  the  anterior,  posterior,  and 
lateral  ligaments  ;  and  it  communicates  with  the  synovial  membrane 
that  lubricates  the  articulations  between  the  os  calcis  and  the  astragalus 
on  the  one  hand,  and  the  cuboid  and  scaphoid  on  the  other.  It  also 
sometimes  supplies  the  two  upper  facets  between  the  os  calcis  and 
astragalus. 

Directions. — The  anterior  and  posterior  ligaments  should  be  incised 
in  order  to  expose  the  synovial  membrane ;  and,  thereafter,  these  and 
the  lateral  ligaments  should  be  cut  away.  This  will  effect  the  separa- 
tion of  the  tibia ;  and  the  next  group  of  ligaments  may  then  be 
examined. 

The  following  ligaments  can  hai'dly  be  classified  as  belonging  specially 
to  any  one  articulation  or  set  of  articulations.  For  the  most  part  they 
bind  together  the  series  of  tarsal  bones,  and  also  serve  to  bind  these  to 
the  metatai-sal  bones. 

The  Astragalo-metatarsal  Ligament. — This  is  a  flat,  radiating  ligament. 


DISSECTION    OF    THE    POSTERIOR    LIMB.  O'i) 

situated  on  the  inner  side  of  the  tarsus.  Its  fibres  are  attached  above 
to  the  tubercle  on  the  inner  side  of  the  astragalus  ;  and,  widening  as  it 
descends,  it  becomes  inserted  into  the  scaphoid,  cuneiform  magnum, 
and  large  metatarsal  bone. 

The  Calcaneo-metatarsal  or  Calcaneo-cuhoid  Ligavient. — This  is  a 
strong,  cord-like  ligament,  situated  at  the  outer  side  of  the  back  of  the 
hock,  and  attached  to  the  posterior  border  of  the  os  calcis,  the  cuboid, 
and  the  head  of  the  external  small  metatarsal  bone. 

The  Tarso-metatarsal  Ligament  will  be  seen  covering  the  tarsal  bones 
at  the  back  of  the  hock.  It  forms  a  thick  mass  of  fibrous  tissue 
intimately  adherent  to  these  bones  and  to  the  heads  of  the  metatarsal 
bones.  Its  inner  border  is  blended  with  the  lateral  ligament  of  the 
tibio-tarsal  joint;  and  its  outer  with  the  calcaneo-metatarsal  ligament. 
Below  it  is  continued  as  the  siibtarsal  ligament,  which  joins  the  per- 
forans  tendon.  The  anterior  face  of  the  ligament,  where  not  adherent 
to  the  bones,  is  lined  by  synovial  membrane  ;  and  its  posterior  face  is 
similarly  lined  by  the  synovial  membrane  of  the  tarsal  sheath. 

Directions. — At  the  front  of  the  hock  the  point  of  a  scalpel  should  be 
introduced  into  the  articulation  between  the  astragalus  and  the 
scaphoid ;  and  by  cutting  round  the  hock  through  the  three  ligaments 
just  described,  an  attempt  should  be  made  to  separate  the  astragalus 
and  OS  calcis,  as  a  single  piece,  from  the  rest  of  the  tarsal  bones.  Before 
this  can  be  effected,  however,  there  must  be  cut  an  interosseous  ligament, 
which  is  composed  of  short  and  strong  fibres  passing  between  the  os 
calcis  and  astragalus  on  the  one  hand,  and  the  cuboid  and  scaphoid  on 
the  other.  At  the  same  time  the  synovial  membrane  belonging  to  the 
articulations  between  these  two  sets  of  bones  will  be  opened  into.  This 
capsule  communicates  in  front  with  that  of  the  tibio-tarsal  joint,  and  "is 
prolonged  superiorly  between  the  calcis  and  astragalus,  to  lubricate  two 
of  the  facets  by  which  these  bones  come  into  contact.  In  addition,  it 
descends  between  the  cuboid  and  scaphoid  bones,  to  form  a  prolongation 
for  the  anterior  cxiboido-scaphoid  arthrodia." — Chauveau. 

Ligaments  uniting  the  Os  Calcis  and  Astragalus. — There  arc  four  of 
these — a  supemor,  two  lateral,  and  an  interosseous.  The  first  of  these  is 
composed  of  fibres  passing  between  the  two  bones  above  their  surfaces 
of  contact ;  the  lateral  ligaments  pass  between  them  on  each  side  ;  while 
the  interosseous  ligament  cannot  be  seen  in  its  entirety,  as  it  jDasses 
between  the  rough  impressions  on  the  surfaces  of  apposition  of  the 
bones,  and  must  be  cut  before  these  can  be  separated. 

Directions.— Ktiewtion  should  next  be  turned  to  the  following  liga- 
ments, which  bind  together  the  other  four  tarsal  bones. 

The  Anterior  Ciiboido-scaphoid  Ligament  is  of  small  size,  and  passes 
between  the  two  bones  from  which  it  is  named,  above  the  entrance  to 
the  canal  by  which  the  perforating  metatarsal  artery  passes  through  the 


90  THE  ANATOMY  OF  THE  HORSE. 

hock.  The  same  bones  are  joined  by  an  interosseous  ligament,  which 
forms  the  roof  of  that  canal. 

The  Anterior  Cuboido-cunean  Ligament  connects  the  cuboid  and  cunei- 
form magnum  bones  below  the  entrance  to  the  above-mentioned  vas- 
cular canal ;  and  an  interosseous  cuboido-cunean  ligament  forms  the  floor 
of  the  canal. 

The  Scapkoido-cunean  Interosseous  Ligament  joins  the  scaphoid  and 
two  cuneiform  bones. 

The  Intercunean  Ligament  passes  between  the  two  cuneiforms. 

These  and  the  other  interosseous  ligaments  are  concealed  in  the 
interstices  between  the  different  bones  which  they  bind  together,  and 
cannot  be  fully  seen  until  the  bones  are  separated. 

Synovial  Membranes. — "There  is  a  proper  synovial  membrane  for  the 
facets  by  which  the  scaphoid  and  cuneiform  magnum  bones  corre- 
spond; this  synovial  membrane  belongs  also  to  the  two  cuboido- 
scaphoid,  and  posterior  cuboido-cunean  arthi'odise.  The  anterior 
cuboido-scaphoid  diarthrosis  receives  a  prolongation  from  the  synovial 
membrane  between  the  os  calcis  and  astragalus  on  the  one  hand, 
and  the  cuboid  and  scaphoid  on  the  other.  The  play  of  the 
anterior  cuboido-cunean,  and  inter-cunean  facets  is  facilitated  by  two 
prolongations  of  the  tarso-metatarsal  synovial  membrane." — Chauveati. 

THE    TARSO-METATARSAL    ARTICULATION. 

An  Interosseous  Ligament  binds  the  heads  of  the  metatarsal  bones  to 
the  tarsal  bones  with  which  these  articulate,  and  the  union  is  further 
secured  by  many  of  the  ligaments,  already  dissected,  which,  though 
they  belong  to  the  hock,  have  points  of  insertion  into  the  heads  of  the 
metatarsal  bones. 

Synovial  Membrane. — This  not  only  supplies  the  tarso-metatarsal  joint, 
but  also  ascends  between  the  two  cuneiforms,  and  into  the  anterior  facet 
between  the  cuboid  and  cuneiform  magnum.  It  also  descends  into  the 
articulations  between  the  large  and  small  metatarsal  bones. 

Directions. — For  a  description  of  the  remaining  joints  of  the  hind  limb 
(except  the  hip),  turn  to  the  description  of  the  corresponding  articula- 
tions of  the  fore  limb  (page  50).  The  hip-joint  is  described  with  the 
pelvis,  at  page  338. 


DISSECTION    OF    THE    POSTERIOR    LIMB. 


91 


TABULAR  VIEW  OF  THE  MUSCLES  IN   THEIR  ACTION  ON  THE  JOINTS 
OF  THE  HIND  LIMB. 


Flexors . 


Extensors 


Hip. 

/Sartorius. 
I  Pectineus. 
1  Psoas  magnus. 
J  niacus. 

i  Tensor  vaginne  femoris. 
I  Rectus  femoris. 
\Rectus  parvus  (?) 
/  Semimembranosus. 
.)  Quadratus  femoris, 
\  Middle  gluteus. 

V  Obturator  externus. 
/  Superficial  gluteus. 
)  Biceps  femoris  (anterior  half). 

^  Middle  gluteus. 

V  Deep  gluteus. 

{Sartorius. 
Gracilis. 
Adductor  magnus. 
Semimembranosus. 
Pectineus. 
Adductor  parvus. 
(  Sartorius. 
t  Gracilis. 
^Deep  gluteus. 
I  Adductor  parvus. 
Quadratus  femoris. 
Psoas  magnus. 
Rotators  outwards  <  Iliacus. 

1  Obturator  externtis. 
Obturator  internus. 
Pyrif  ormis. 
^Gemelli. 

Stifle. 
(  Biceps  femoris  (posterior  half). 
<.  Semitendinosus. 
(.  Popliteus. 
/"  Vastus  internus. 
.)  Vastus  externus. 
j  Rectus  femoris. 

V  Biceps  femoris  (anterior  half). 
—  Biceps  femoris  (jiosterior  half). 

(  Semitendinosus. 
)  Popliteus. 

Hock. 


Rotators  inwards 


Extensors 


Rotator  outwards 
Rotators  inwards 


Flexors 


Extensor  pedis. 
Peroneus. 
Flexor  metatarsi. 


Extensors 


(Gastrocnemius. 
Soleus. 
Flexor  perforatus. 
Flexor  perforans. 
Flexor  accessorius. 


Flexors  . 


'  Flexor  perforatus. 
Flexor  perforans. 
'  Flexor  accessorius. 


Extensors 


P.\STERN. 


Flexor  perforatus. 
Flexor  perforans. 
.  Flexor  accessorius. 


(  Extensor  pedis. 
<  Peroneus, 
I  Extensor  brevis. 


(  Extensor  pedis. 
<  Peroneus. 
(Extensor  brevis. 


Flexors . 


Flexor  perforans. 
Flexor  accessorius, 


Coffin-joint. 

Extensors 


(  Extensor  pedis. 
<  Peroneus. 
(  Extensor  br  evis. 


92 


THE    AKATOMY    OF    THE    HORSE. 


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DISSECTION    OF    THE    POSTERIOR    LIMB. 


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CHAPTER     III. 

DISSECTION   OF   THE   BACK   AND   THORAX. 

The  dissection  of  the  thorax  should  be  begun  at  the  same  time  as  that 
of  the  outer  scapular  region  (see  page  8). 

THE    CHEST-WALL    AND    BACK, 

Directions. — The  portion  of  skin  remaining  on  the  chest-wall  and  loins 
should  be  removed,  the  operation  being  commenced  at  an  incision 
carried  along  the  middle  line,  from  the  withers  to  the  croup.  The  cut- 
aneous nerves  must  then  be  sought,  after  which  the  other  structures 
are  to  be  taken  up  in  the  order  of  their  description. 

Cutaneous  Nerves.  In  the  back  these  are  derived  from  the  dorsal 
nerves.  One  set  of  branches  appears  close  to  the  spinous  processes  ; 
and  another  a  few  inches  outwards,  along  the  course  of  the  longissimus 
dorsi  muscle.  Both  of  these  are  derived  from  the  superior  primary 
branches  of  the  dorsal  nerves.  Over  the  sides  of  the  chest  the  cutaneous 
nerves  are  derived  from  the  perforating  bi-anches  of  the  intercostal 
nerves,  which  are  dissected  with  the  abdominal  muscles  (see  page  288). 

In  the  loins  the  cutaneous  nerves  are  derived  from  the  superior 
primary  branches  of  the  lumbar  nerves,  and  the  most  posterior  of  them 
are  continued  backwards  to  the  skin  over  the  gluteal  region. 

The  Panniculus  Carnosus  (Plate  38).  This  is  an  extensive  muscle 
adherent  to  the  deep  surface  of  the  skin  over  a  large  part  of  the  abdo- 
men, thorax,  and  shoulder.      It  is  fully  described  at  page  287,  which  see. 

Directions. — The  panniculus  should  now  be  entirely  removed,  begin- 
ning at  its  upper  border. 

The  Latissimus  Dorsi.  This  muscle  is  partly  described  at  pages 
9  and  14,  in  connection  with  the  dissection  of  the  fore  limb.  It 
arises  by  a  broad  aponeurotic  tendon  from  the  series  of  vertebral  spines, 
beginning  about  the  4th  dorsal,  and  extending  back  to  the  last  Iximbar. 
This  tendon  is  not  well  defined  at  its  inferior  border,  where  it  is 
adherent  to  the  ribs,  and  blends  with  the  oblique  muscles  of  the 
abdomen.  Posteriorly  the  tendon  becomes  continuous  with  the  gluteal 
fascia.  The  tendon  is  succeeded  by  a  thick  muscular  portion,  which 
contracts  and  passes  to  the  inner  side  of  the  fore  limb,  where  it  becomes 
inserted  into  the  internal  tubercle  of  the  humerus. 

Action. — It  is  a  flexor  and  an  inward-rotator  of  the  shoulder-joint. 


DISSBCTIOX    OF    THE    BACK    AND    THORAX.  95 

The  Serratus  Magnus  (Plate  4).  This  muscle  will  be  seen  here,  as 
left  by  the  dissector  of  the  fore  limb.  The  student  should  notice  its 
mode  of  origin  from  the  ribs  (see  page  7),  and  then  carefully  remove  it. 

Directions. — The  latissimus  dorsi  must  now  be  removed,  beginning 
below,  where  its  muscular  portion  was  cut  by  the  dissector  of  the  fore 
limb.  This  operation  must  be  conducted  with  care,  in  order  to  leave 
intact  the  anterior  and  posterior  serratus  muscles,  whose  thin  tendons 
are  adherent  to  that  of  the  latissimus.  Indeed,  over  the  last  ribs,  in  an 
old  subject,  it  will  be  found  impossible  to  separate  the  latissimus  from 
the  underlying  serratus,  and  the  former  may  there  be  cut  off. 

The  Serratus  Posticus  (Plate  20).  (This  and  the  succeeding 
muscle  are  described  together  by  Percivall,  under  the  name  superficialis 
costarum.  It  corresponds  to  the  serratus  2^osticus  inferior  of  man.) 
This  muscle  is  provided  with  an  aponeurotic  tendon,  by  which  it 
arises  from  the  summits  of  the  vertebral  spines  from  the  11th  dorsal  to 
the  2nd  lumbar.  The  inferior  border  of  the  tendon  has  a  muscular 
fringe  with  eight  or  nine  distinct  slips,  which  are  inserted  into  the 
posterior  borders  and  outer  surfaces  of  the  same  number  of  ribs  at  the 
end  of  the  series. 

Actio7i. — It  is  a  muscle  of  expiration. 

The  Serratus  Anticus  (Serratus  j^osticus  sujjerior  of  man)  (Plate  20). 
This  muscle  is  partly  covered  by  the  preceding,  whose  three  anterior 
slips  should  therefore  be  carefully  removed,  as  has  been  done  in  Plate 
20.  It  repeats  the  form  of  the  posticus,  having  a  thin,  translucent 
aponeurotic  tendon,  which,  in  front,  is  confounded  with  the  splenitis. 
By  the  upper  border  of  this  tendon  it  arises  from  the  summits  of  the 
dorsal  spines  from  the  2nd  or  3rd  to  the  13th.  The  inferior  border  of 
the  tendon  is  succeeded  by  the  fleshy  portion  of  the  muscle,  and  this  is 
inserted  into  the  anterior  borders  and  outer  surfaces  of  the  ribs  from  the 
5th  to  the  13th  inclusive. 

Action. — To  assist  in  inspiration. 

Directions. — The  two  muscles  just  described  must  be  removed  in 
order  to  expose  the  next  layer ;  and  this  is  to  be  done  by  incising  the 
aponeurotic  portion  of  each  hoi-izontally,  an  inch  or  two  above  its  point 
of  junction  with  the  muscular  portion.  The  portions  above  the  incision 
can  then  without  difficulty  be  stripped  vipwards  from  the  sui-face  of 
the  longissimus  dorsi.  The  lower  portions  must  next  be  dissected 
dowTiwards  in  order  to  expose  the  transversalis  costarum.  In  doing 
this,  it  will  be  found  that  a  fibrous  septum  passes  from  the  aponeurosis 
of  the  serratus  anticus  near  its  lower  border,  and,  penetrating  between 
the  two  muscles  now  exposed,  becomes  attached  to  the  ribs.  This  must 
be  cut,  and  the  muscular  slips  of  the  seiTati  must  be  carefully  raised 
from  the  transversalis  costai'um. 

Vessels  and  Nerves.       A  set  of  nerves  will  be  found  at  the  inner 


96  THE  ANATOMY  OF  THE  HORSE. 

edge  of  the  longissiinus  doi-si,  and  another  perforating  its  substance. 
Both  sets  are  derived  from  the  superior  primary  branches  of  the  dorsal 
or  himbar  nerves. 

The  arteries  and  veins  are  branches  of  the  doi'so-spinal  divisions  of  the 
intei-costal  or  lumbar  vessels. 

The  Transversalis  Costarum  (Plate  21).  (This  muscle  corresponds 
to  the  ilio-costalis  and  musmlns  accessorius  of  man.)  This  is  a  composite 
muscle  extending  across  the  entire  series  of  ribs,  being  five  or  six  inches 
removed  from  the  spine  posteriorly,  but  close  to  it  in  front.  Its  fibres 
are  directed  forwards  and  slightly  downwards,  and  it  possesses  two 
series  of  tendons.  One  set,  forming  slips  of  origin,  is  concealed  at  the 
upper  edge  of  the  muscle ;  the  other,  serving  as  slips  of  insertion,  is 
visible  at  its  lower  edge.  By  the  upper  set  of  tendons  it  aHses  from  the 
transverse  processes  of  the  first  two  lumbar  vei'tebra;,  and  from  the 
anterior  borders  of  the  ribs.  By  the  lower  set  of  tendons  it  is  imerted 
into  the  hinder  edges  of  the  ribs  anterior  to  the  llth,  and  to  the  trans- 
verse process  of  the  last  cervical  vertebra. 

Action. — To  pull  the  ribs  backwards,  and  thus  assist  in  expiration. 
Both  muscles  acting  together  may  also  assist  in  extending  the  spine ; 
or  acting  singly,  they  may  incline  it  laterally. 

The  LoxGissiMUS  Dorsi  (Plate  21).  This  is  the  longest  and 
strongest  muscle  in  the  body,  and  it  is  also  the  most  complex.  It 
extends  along  the  spine,  from  the  sacrum  to  the  neck.  In  the  loins  it 
forms  a  great  muscular  and  tendinous  mass  (the  common  mass  of  man) ; 
and  anteriorly  it  is  bifurcate,  the  trachelo-mastoid  and  complexus 
muscles  getting  origin  between  its  two  branches.  Its  fibres  arise  from 
the  sacral  sui-face  of  the  ilium  between  the  crest  and  the  sacro-iiiac  joint, 
and  from  a  strong,  glistening  fascia  covering  the  surface  of  the  muscle, 
this  fascia  being  fixed  to  the  lumbar  and  dorsal  spines,  or  to  the  supra- 
spinous ligament.  Its  fibres  are  inserted  into  the  lumbar  transverse  and 
articular  processes,  the  dorsal  transvei-se  processes,  and  the  ribs  as  far 
outwards  as  the  edge  of  the  transversalis  costarum.  About  the  5th  rib  it 
divides ;  and  the  lower  branch,  continuing  the  outer  series  of  attachments, 
is  inserted  into  the  ribs,  and  the  transverse  processes  of  the  first  four 
dorsal  and  last  four  cei-vical  vertebrai ;  while  the  upper  division,  getting 
many  new  fibres  from  the  first  four  dorsal  spines,  becomes  inserted  into 
the  spinous  processes  of  the  four  cervical  vertebi*a3  in  front  of  the  last. 

Action. — Acting  with  the  opposite  muscle,  it  is  the  great  extensor  of 
the  dorso-lumbar  portion  of  the  spine,  being,  in  this  respect,  the  chief 
antagonist  of  the  sublumbar  and  abdominal  muscles.  By  its  costal 
attachments  it  may  also  assist  in  expiration.  By  its  cei-vical  attach- 
ments it  raises  the  neck.  Acting  singly,  it  inclines  the  spine  to  the 
side  of  the  acting  muscle. 

Retractor  Cost^^e  (Plate  45).      This  is  a  small  triangular   muscle 


DISSECTION    OF    THE    BACK    AND    THORAX,  97 

■which  hes  under  cover  of  the  last  slip  of  the  serratus  posticus.  It  is 
tliin  and  aponeurotic  at  its  upper  edge,  where  it  arises  from  the  first 
two  or  three  lumbar  transverse  processes.  The  remainder  of  the  muscle 
is  fleshy,  and  it  is  inserted  by  its  anterior  edge  into  the  posterior  boi-der 
of  the  last  rib.  Its  lower  edge  is  parallel  to  the  highest  fibres  of  the 
internal  oblique  muscle  of  the  abdomen,  and  it  is  generally  described  as 
a  part  of  that  muscle. 

Action. — To  assist  in  expiration. 

Directions. — Two  sets  of  muscles  lie  under  cover  of  the  longissimus 
dorsi,  viz.,  the  semispinalis  of  the  back  and  loins,  and  the  levatores 
costarum.  A  segment  of  the  longissimus,  from  the  13th  to  the  17th 
rib,  should  be  excised  after  the  fashion  of  Plate  21;  or  if  it  be  desired 
to  expose  the  whole  of  each  series,  the  longissimus  dorsi  must  be 
entirely  removed. 

The  Levatores  Costarum  (Plate  21).  These  form  a  series  of  small 
mxiscles,  each  occupying  the  extreme  upper  part  of  an  intercostal  space, 
and  at  that  point  taking  the  place  of  the  external  intercostal  muscle. 
Each  arises  from  the  transverse  process  of  a  dorsal  vertebra ;  and 
passing  downwards  and  backwards,  it  expands,  and  becomes  inserted 
into  the  outer  surface  of  the  rib  posterior  to  the  vertebra  from  which  it 
takes  origin.  In  the  first  two  or  three  spaces  the  muscles  are  rudi- 
mentary or  absent. 

Action. — To  assist  in  inspiration. 

The  Semispinalis  of  the  back  and  loins  (Plate  21).  This  is  a  com- 
posite muscle,  covering  the  sides  of  the  vertebral  spines  from  the  sacrum 
to  the  neck,  and  consisting  of  numerous  fasciculi  directed  obliquely 
upwards  and  forwards.  Anteriorly  these  fasciculi  are  in  series  with 
the  semi-spinalis  colli,  and  posteriorly  with  the  curvator  coccygis.  The 
fasciculi  take  m-igin  from  the  lateral  lip  of  the  sacrum,  from  the  articular 
tubercles  of  the  lumbar  vertebrae,  and  from  the  transverse  processes  of 
the  dorsal  vertebrae.  They  become  inserted  into  the  vertebral  spines, 
each  fasciculus  being  inserted  into  the  3rd  or  4th  vertebra  anterior  to  the 
one  from  which  it  takes  origin.  In  the  forepart  of  the  dorsal  region  (Fig. 
18,  page  156)  the  insertion  is  into  the  sides  of  the  spines,  but  elsewhere 
it  is  into  or  near  the  summits  of  the  processes. 

Action. — It  is  an  extensor  or  a  lateral  flexor  of  the  spine,  according  as 
the  right  and  left  muscles  act  together  or  singly. 

Directions. — Clean  the  outer  surfaces  of  a  few  of  the  external  inter- 
costal muscles  about  the  middle  of  the  series,  and  at  the  side  of  the 
sternum  define  the  lateralis  sterni  muscle. 

The  Lateralis  Sternl  This  is  a  thin,  flat  muscle,  a  few  inches 
broad.  Arising  from  the  outer  surface  of  the  1st  rib  above  its  car- 
tilage, it  passes  obliquely  downwards  and  backwards  over  the  2nd 
chondro-costal  joint,  and  over  the  3rd  and  4th  costal   cartilages,    and 


98  THE  ANATOMY  OF  THE  HORSE. 

is  insei'ted  into  the  side  of  the  stermim.  Fi*equeutly  some  of  its  fibres 
terminate  on  the  3rd  and  4th  costal  cartilages,  or  on  the  aponeurosis 
over  the  internal  intercostal  between  these  cartilages. 

Action. — Acting  from  its  attachment  to  the  1st  rib  as  its  fixed  point, 
the  muscle  will  exert  a  feeble  inspiratory  action. 

The  External  Intercostal  Muscles  (Plate  21).  Each  muscle  of  this 
set  occupies  an  intercostal  space,  extending  from  near  the  spine  as  far 
as  the  lower  extremities  of  the  ribs.  The  muscular  fibres  of  each  are 
fixed  by  their  extremities  to  the  margins  of  the  ribs  that  bound  the 
intercostal  space.  They  pass  obliquely  downwards  and  backwards  ;  and 
may  be  considered  as  having  their  point  of  ongin  from  the  anterior  rib, 
and  their  insertion  into  the  posterior  rib. 

Directions. — In  one  or  two  of  the  spaces  the  external  intercostal  should 
be  removed  (see  Plate  21)  in  order  to  expose  the  internal  muscle,  which 
will  readily  be  distinguished  by  the  different  direction  of  its  fibres. 

The  Internal  Intercostal  Muscles  (Plate  21).  These  equal  in 
number  the  external  set,  one  being  lodged  in  each  intercostal  space. 
They  differ  from  the  external  set  in  that  they  are  prolonged  beyond 
the  lower  extremities  of  the  ribs  to  fill  the  interspaces  of  the  costal 
cartilages,  while  in  the  extreme  upper  part  of  the  intercostal  spaces  they 
are  absent  or  much  reduced  in  thickness.  They  differ,  moreover,  in  the 
direction  of  their  fibres,  Avhich  is  oblique  downwards  and  forwards ;  and 
each  may  be  viewed  as  having  its  origin  from  the  postei'ior  rib  and 
cartilage,  and  its  insertion  into  the  anterior  rib  and  cartilage,  of  the 
space  that  it  occupies.  The  inner  surface  of  each  is  lined  by  pleura, 
but  at  present  no  attempt  need  be  made  to  expose  this. 

Action  of  the  intercostal  muscles. — The  external  set  and  the  inter- 
cartilaginous  portions  of  the  internal  set  are  muscles  of  inspiration. 
The  interosseous  portions  of  the  internal  set  are  muscles  of  expiration. 

Directions. — In  a  few  of  the  intercostal  spaces  the  vessels  and  nerves 
should  be  exposed.  They  will  be  found  at  the  hinder  edge  of  the  rib, 
and  should  l)e  followed  upwards  and  downwards. 

Intercostal  Arteries.  There  are  seventeen  intercostal  arteries  on 
each  side,  one  for  each  space.  The  first  is  derived  from  the  superior 
cervical  artery  ;  the  second,  third,  and  fourth  from  the  dorsal  artery  or  its 
subcostal  branch  ;  and  the  remaining  thirteen  from  the  posterior  aorta. 
Their  points  of  origin  will  be  seen  in  the  dissection  of  the  cavity  of 
the  thorax.  Each  vessel  on  gaining  the  upper  extremity  of  the  inter- 
costal space  gives  off  a  large  dorso-spinal  branch,  and  then  descends 
behind  the  rib,  with  the  vein  and  nerve.  The  dorso-spinal  artery  sends 
a  branch  into  the  spinal  canal  by  the  vertebral  foramen,  and  is  then 
expended  in  the  muscles  occupying  the  costo-vertebral  groove  at  the  side 
of  the  dorsal  spines.  In  the  intercostal  space  the  intercostal  artery  is 
accompanied  by  a  vein  and  nerve,  the  vein  being  in  front,  and  the  nerve 


DISSECTION    OF    THE    BACK    AND    THORAX.  99 

posterior.  In  the  upper  third  of  the  space  the  vessels  descend  between 
the  outer  and  inner  muscles,  and  rest  in  the  groove  at  the  posterior  edge 
of  the  rib.  For  the  rest  of  their  course  they  are  under  cover  of  the 
hinder  edge  of  the  rib,  and,  generally,  between  the  inner  muscle  and  the 
pleura ;  but,  here  and  there,  slips  of  the  inner  muscle  may  pass  between 
the  vessels  and  the  pleura.  At  the  lower  extremities  of  the  intercostal 
spaces  the  arteries  behave  as  follows  : — The  first  six  (or  seven)  anastomose 
with  ascending  branches  from  the  internal  thoracic  artery  ;  the  remainder 
as  far  as  the  thirteenth  anastomose  with  similar  branches  from  the 
asternal  artery ;  and  the  last  four  run  into  the  abdominal  wall  and  are 
expended  in  its  muscles,  anastomosing  with  the  abdominal  and  circumflex 
iliac  arteries.  In  their  descent  the  intercostal  arteries  give  off  costal, 
pleural,  muscular,  and  cutaneous  branches. 

The  Intercostal  Veins  accompany  the  arteries.  On  the  left  side 
the  first  joins  the  superior  cervical  vein,  the  next  ten  or  eleven  join 
the  left  dorsal  vein,  and  the  last  five  or  six  the  great  vena  azygos. 
On  the  right  side  the  first  joins  the  superior  cervical  vein,  the  next 
three  join  the  dorsal  vein,  and  the  remaining  thirteen  the  great  vena 
azygos. 

The  Dorsal  Nerves.  There  are  eighteen  dorsal  nerves,  one  emerging 
by  the  intervertebral  foramen  behind  each  dorsal  vertebra.  Each  divides 
in  the  foramen  to  form  a  superior  and  an  inferior  primary  branch.  The 
superior  primary  branch  supplies  the  muscles  in  the  costo-vertebral 
groove,  and  the  superjacent  skin.  The  inferior  primary  branch  of  the 
1st  nerve,  after  detaching  a  very  slender  intercostal  twig,  joins  the 
brachial  plexus.  The  2nd  nerve  gives  a  slender  branch  to  the  brachial 
plexus,  and  is  continued  as  the  intercostal  nerve  of  the  second  space. 
The  inferior  primary  branches  of  the  succeeding  nerves,  except  the  last, 
are  directly  continued  as  intercostal  nerves.  The  inferior  primary 
branch  of  the  last  (18th)  dorsal  nerve  descends  behind  the  last  rib  (see 
pages  292  and  324). 

The  Intercostal  Nerves. — These  accompany  the  intercostal  vessels,  and 
terminate  thus  : — The  1st  intercostal  nerve  is  very  slender  and  does  not 
reach  the  bottom  of  the  space  ;  the  six  nerves  behind  the  1st  perforate 
the  pectoral  muscles  and  become  cutaneous  at  the  side  of  the  sternum ; 
the  others  (ten)  are  continued  beyond  the  lower  extremities  of  the  inter- 
costal spaces  to  be  distributed  in  the  abdominal  wall. 

The  intercostal  nerves  give  branches  to  the  muscles  of  the  same  name, 
and  about  the  middle  of  the  intercostal  space  each  gives  off  a  large  per- 
forating branch  (lateral  cutaneous  of  man)  to  supply  the  panniculus  and 
overlying  skin. 

The  Lumbar  Nerves  and  Vessels.  The  superior  primary  branches  of 
these  nerves  (six  in  number)  have  a  distribution  in  the  loins  analagous 
to  the  corresponding  branches  of  the  dorsal  nerves  in  the  back.     They 


100  THE  ANATOMY  OF  THE  HORSE. 

supply  muscular  branches  to  the  muscles  over  the  lumbar  transverse 
processes,  and  cutaneous  twigs  to  the  skin  of  the  loins  and  croup. 
Branches  of  the  kxmbar  arteries  and  veins  accompany  these  nerves. 
Each  artery  sends  a  spinal  branch  through  the  intervertebral  foramen. 

THE    CAVITY    OF    THE    THORAX. 

Directions.— In  order  to  expose  the  thoracic  cavity,  the  chest -walls 
must  be  in  part  removed  ;  and  it  is  most  convenient,  in  the  first  place, 
to  make  the  opening  on  the  left  side.  The  trunk  should  be  allowed  to 
remain  in  the  suspended  position.  If  the  diapliragm  is  intact,  and  if  no 
opening  quite  through  the  chest-wall  has  been  made  in  the  previous 
dissection,  then  the  first  step  should  be  to  perforate  one  of  the  intercostal 
spaces  with  the  finger  or  a  blunt  instrument.  This  is  to  be  done  in 
order  to  allow  the  lungs  to  collapse ;  and  a  sharp  instrument  must  not 
be  used,  lest  the  surface  of  the  lung  might  be  injured.  As  soon  as  the 
finger  or  instrument  is  withdrawn  from  the  aperture,  the  air  will  be 
heard  to  rush  in  and  fill  the  pleural  cavity,  which  was  previously 
occupied  by  the  distended  lung.  This  is  precisely  what  occurs  when 
the  chest-wall  is  perforated  in  the  living  animal,  in  which,  in  health,  the 
outer  surface  of  the  lung  is  closely  applied  to  the  inner  surface  of  the 
wall,  following  it  in  all  its  movements.  The  lung  is  kept  in  this  dis- 
tended state  by  the  atmospheric  pressure,  which  operates  on  the  air 
passages  in  the  interior  of  the  lung,  but  not  on  its  exterior,  where  the 
pressure  is  borne  by  the  chest-wall ;  and  the  lung  is  kept  thus  distended, 
in  opposition  to  a  strong  natural  tendency  to  contract,  which  it  possesses 
in  virtue  of  the  large  amount  of  elastic  tissue  in  its  structure.  But 
w^hen  the  wall  of  the  chest  is  perforated,  the  pressure  of  the  atmosphere 
becomes  exerted  on  the  exterior  as  well  as  the  interior  of  the  lung,  and 
the  imopposed  elasticity  of  the  lung  texture  then  comes  into  play. 

By  means  of  the  saw  and  bone-forceps,  the  ribs,  except  the  first  and 
a  few  at  the  end  of  the  series,  are  to  be  removed,  the  upper  section 
being  made  a  few  inches  below  the  head  of  each  rib,  and  the  lower  a 
little  above  the  chondro-costal  articulation. 

Form  and  Boundaries  of  the  Cavity  (Plates  22  and  25). — If  the  con- 
tained organs  were  removed  from  the  thorax,  and  a  cast  were  taken  of  its 
interior,  it  would  be  found  to  have  an  irregularly  conical  form,  but  the 
symmetry  of  the  cone  is  largely  departed  from.  The  base  of  the  cone 
is  formed  by  the  diaphragm,  which,  viewed  from  the  thoracic  side,  is 
markedly  convex  like  the  roof  of  a  dome.  The  plane  of  attachment  of 
the  diaphragm  slopes  downwards  and  forwards,  so  that  the  antero- 
posterior measurement  of  the  cavity  is  much  less  below  than  above. 
Moreover,  as  the  diaphragm  is  dome-shaped,  this  measurement  is  less 
when  taken  from  its  centre  than  from  its  sides.  It  is  in  consequence  of 
this  configuration  of  the  diaphragm,  that  the  liver,  the  stomach,  and 
other  abdominal  organs  lie  under  cover  of  the  ribs.     The  vei-tex  of  the 


DISSECTION    OP    THE    BACK   AND    THORAX. 


101 


cone  lies  in  front,  and  is  bounded  by  the  body  of  the  1st  dorsal  vertebra 
above,  and  at  the  sides  by  the  1st  ribs,  which  meet  below.  The 
trachea,  the  oesophagus,  the  bloodvessels  of  the  fore  limb  and  head, 
and  many  important  nerves  ai-e  transmitted  through  this  opening.  On 
transverse  section,  the  thorax  is  not  circular,  as  a  cone  is,  but  gives  a 
heart-shaped  outline.  It  looks  as  if  it  had  been  squeezed  laterally  ;  and 
it  might  be  described  as  having  a  roof,  formed  by  the  dorsal  vertebrae 
and  the  ribs  as  far  as  their  angles ;  a  floor,  much  less  extensive,  formed 
by  the  sternum  ;  and  lateral  walls,  formed  by  the  ribs  and  intercostal 
muscles. 

Contents  of  the  Cavity. — In  point  of   size,  the    lungs  are   the   most 
important  organs  in  the  thorax,  the  heart  coming  next.      Besides  these. 


1.  Chest-wall;  2.  Pleural  cavity  or  sac;  3.  Lung;  4.  Mediastinal  pleura  (parietal);  5.  Costal 
pleura  (parietal) ;  6.  Pulmonic  pleura  (visceral);  A.  (No.  3)  Abdominal  cavity  ;  D.(No.  3)  Diaphragm  ; 
P.  (No.  1)  Pericardial  sac;  R.  (No.  1)  Root  of  lung  ;  V.  (No.  3)  Fold  of  right  pleural  membrane 
enveloping  posterior  vena  cava. 

the  cavity  lodges  the  main  arterial  and  venous  trunks,  the  thoracic  duct, 
the  trachea,  the  oesophagus,  and  many  important  nerves,  all  of  which 
will  be  examined  in  due  course. 

The  Pleurae.  Each  half  of  the  thorax  possesses  a  serous  membrane 
termed  the  pleura.  Like  other  serous  membranes,  the  pleura  is  arranged 
in   the  form  cf  a  shut  sac,    and  consists   of  a  visceral  and  a  parietal 


102  THE  ANATOMY  OF  TUE  HORSE. 

portion.  The  visceral  pleura  is  that  which  invests  the  lung,  and  it  is 
therefore  termed  the  pulmonic  pleura.  Around  the  root  of  the  hmg  it  is 
continuous  with  the  parietal  portion.  The  panetal  pleura  lines  the 
walls  of  the  chest  on  the  side  to  which  it  belongs.  It  covers  the  inner 
surface  of  the  ribs  and  intercostal  muscles,  forming  the  costal  pleura ;  it 
is  spread  over  the  anterior  surface  of  the  diaphragm,  constituting  the 
diaphragmatic  pleura ;  and  towards  the  middle  line  of  the  cavity  it, 
together  with  the  corresponding  layer  of  the  opposite  side,  forms  a 
vertical  septum  termed  the  mediastinum.  This  is  the  mediastinal 
pleura.  Behind  the  root  of  the  lung  a  double  fold  of  pleura,  termed 
the  ligamentum  latum  jndmonis,  is  prolonged  along  the  mediastinum  to 
the  diaphragm.  On  the  right  side  of  the  chest  the  pleura  forms  a  special 
fold  that  includes  between  its  two  layers  the  posterior  vena  cava  and 
the  right  phrenic  nerve.  All  these  differently  named  divisions  of  the 
pleura  are  continuous  the  one  Avith  the  other  ;  and  they  unite  to  form  a 
close  sac  termed  the  pleural  cavity.  This  disposition  of  the  pleura  will  be 
more  readily  understood  by  reference  to  the  accompanying  diagrams  (page 
101),  the  first  of  which  represents  the  arrangement  of  the  membrane  at  the 
root  of  the  lung,  the  second  in  front  of,  and  the  third  behind,  that  point. 

These  diagrams,  it  is  to  be  obsei-ved,  however,  are  not  true  to  nature ; 
for,  whereas  they  show  a  distinct  pleural  cavity,  in  the  living  healthy 
animal  that  cavity  has  only  a  potential  existence,  the  pulmonic,  being 
everywhere  in  contact  with  the  visceral,  pleura.  But  when  air  is 
admitted  to  the  cavity,  or  when  inflammatory  or  other  effusions  are 
poured  out  from  the  surface  of  the  membrane,  the  parietal  and  the 
visceral  pleura  become  separated,  and  the  cavity  comes  to  have  an  actual 
existence.  The  free  sui-ftice  of  the  healthy  pleura  is  exquisitely  smooth, 
and  is  lubricated  by  a  sparing  amount  of  serous  fluid,  which  gives  it  a 
glistening  aspect.  Its  function  is  to  facilitate  the  movements  of  the 
lung  on  the  walls  of  the  chest  during  respiration.  When,  in  inflamma- 
tion of  the  membrane  (plem-isy),  it  loses  its  smoothness  and  becomes 
dry,  these  movements,  which  normally  give  rise  to  no  sensation,  are 
attended  with  the  most  acute  pain.  In  structure,  the  pleura,  like  other 
serous  membranes,  comprises  a  single  layer  of  endothelial  cells  forming 
the  free  surface  of  the  membrane,  and  a  sub-endothelial  layer  of  fibrous 
connective-tissue  supporting  the  bloodvessels,  nerves,  and  lymphatics. 

The  Mediastinum.  This,  as  has  already  been  said,  is  a  septum 
formed  towards  the  mesial  plane  of  the  chest  by  the  approximation  of 
two  layers  of  pleura,  one  from  each  sac.  At  some  points  the  right  and 
left  layers  are  in  close  contact,  as,  for  example,  in  front  of  the  heart  in 
a  lean  subject ;  but  at  other  points  the  layers  are  piished  apart  by  organs 
included  between  them.  The  largest  of  these  organs  is  the  heart, 
opposite  which  the  right  and  left  layers  of  the  mediastinum  are  distant 
four  or  five  inches  from  one  another.     In  the  foetus  of  the  horse,  and 


DISSECTION    OF    THE    BACK    AND    THORAX.  103 

throughout  adult  life  in  some  animals,  the  mediastinum  is  a  complete 
imperforate  septum,  there  being  no  communication  between  the  right 
and  left  pleural  sacs ;  but  in  the  adult  horse  the  mediastinum  immedi- 
ately behind  the  heart  is  cribriform  or  lace-like,  and  through  the  aper- 
tures which  exist  here,  a  pleural  effusion  formed  on  one  side  passes 
readily  through  to  the  other. 

The  heart,  contained  within  its  pericardial  sac,  is,  as  has  already  been 
stated,  the  largest  organ  in  the  mediastinum,  and  it  is  situated  about 
the  centre  of  that  septum.  For  convenience  of  description,  this  division 
of  the  mediastinum  and  the  part  vertically  over  it  may  be  termed  the 
middle  mediastinum;  and  the  portions  before  and  behind  this,  the 
anterior  mediastinum  and  the  2iosterior  mediastinum  respectively.  Adopt- 
ing this  arbitrary  division  of  the  mediastinum,  the  organs  included  in 
it  may  be  tabulated  thus  : — 

In  the  Anterior  Mediastinum. — The  trachea  ;  the  ojsophagus  ;  the 
axillary  and  innominate  arteries  and  their  collateral  branches ;  the 
anterior  vena  cava  and  its  tributaries  ;  the  thoracic  duct ;  the  pneumo- 
gastric,  recurrent,  phrenic,  and  cardiac  nerves ;  the  tracheal  lymphatic 
glands ;  and,  in  the  foetus  and  young  animal,  the  thymus  gland. 

In  the  Middle  Mediastinum. — The  pericardium  and  the  heart ;  the 
common  aorta  and  its  bifurcation  into  anterior  and  posterior  aortfe ;  the 
terminations  of  the  anterior  vena  cava  and  vena  azygos  ;  the  pulmonary 
vessels ;  the  thoracic  duct ;  the  trachea  and  its  bifurcation  into  the 
bronchi  3  the  cesophagus  ;  the  pneumogastric,  phrenic,  and  left  recurrent 
nerves ;  and  the  bronchial  lymphatic  glands. 

In  the  Posterior  Mediastinum. — The  posterior  aorta,  the  vena  azygos, 
the  thoracic  duct,  the  oesophagus,  the  oesophageal  continuations  of  the 
pneumogastric  nerves,  the  left  phrenic  nerve,  and  the  oesophageal 
lymphatic  glands. 

The  posterior  vena  cava,  and  the  right  phrenic  nerve  in  the  latter 
part  of  its  course  are  not  in  the  mediastinum,  being  included  in  a 
special  doubling  belonging  to  the  right  pleural  membrane, 

THE  LUNGS    (PLATBS  22  AND  25). 

The  lungs  are  two  in  number,  and  they  occupy  the  greater  part  of 
the  cavity  of  the  thorax.  As  now  seen,  however,  they  are  collapsed, 
and  occupy  but  a  small  moiety  of  the  cavity,  a  condition  which  makes 
their  examination  more  easy.  Each  lung  appears  to  lie  somewhat  loosely 
in  the  chest ;  but  if  it  be  grasped,  and  an  attempt  be  made  to  remove 
it  bodily,  it  will  be  found  to  be  attached  at  a  point  on  its  inner  surface. 
This,  which  is  termed  the  root  of  the  lung,  is  the  point  where  the  bronchi 
and  vessels  enter  it.  Each  lung  presents  for  examination  two  surfaces, 
three  borders,  a  base,  and  an  apex. 

The  External  (or  costal)  surface  is  much  the  larger  of  the  two.      It  is 


104  THE  AXATOMV  OK  TUE  HORSE. 

smooth  and  convex,  and  in  health  it  is  closely  applied  to  the  chest-wall. 
The  intei'nal  (or  mediastinal)  surface  is  moulded  on  the  mediastinum 
and  the  organs  contained  in  it.  Thus,  it  presents  opposite  the  heart 
a  dejiression  for  the  lodgment  of  that  organ ;  behind  that  point,  and 
near  the  uj)per  limit  of  tlie  surface,  a  longitudinal  groove  for  the  posterior 
aorta ;  and  beneath  that  again  a  second  furrow  parallel  to  the  fii-st  but 
not  so  deep,  which  is  the  impress  left  1)y  the  oisophagus.  This  last 
impression  is  very  faint  on  the  right  lung.  This  surface  also  presents 
the  root  of  the  lung,  which  is  situated  close  behind  and  above  the 
depression  for  the  heart ;  and  the  broad  lif/ament  of  the  lung  (or  ligamen- 
tum  latum  2)idnionis)  already  mentioned.  In  front  of  the  heart,  where 
this  surface  is  applied  to  the  anterior  mediastimim,  it  is  narrow  and  flat. 
The  inner  surface  of  the  right  lung  presents  posteriorly  a  small,  semi- 
detached lobule,  not  present  on  the  left.  The  base  (or  diajyhragmatic 
surface)  is  concave  and  moulded  on  the  diaphragm.  This  surface  on  the 
right  lung  shows  the  base  of  the  small,  semi-detached  lobule,  and  the 
posterior  vena  cava  disappearing  into  the  fissure  between  that  lobule 
and  the  main  mass  of  the  lung.  The  apex  of  the  lung  is  pointed,  and 
lies  at  the  entrance  to  the  chest.  The  sujyerior  (or  vertebral)  border  is 
long,  thick,  and  roimded,  and  it  is  lodged  in  the  costo-vertebral  groove 
at  the  roof  of  the  cavity.  The  infei-ior  (or  sternal)  border  is  short  and 
sharp;  and  opposite  the  heart  it  is  widely  notched,  a  circumstance 
which  allows  the  pericardium  to  be  tapped  at  this  point  without  danger 
of  wounding  the  lung.  The  notch  is  smaller  on  the  right  side.  The 
posterior  (or  diaphragmatic)  border  circumscribes  the  base,  and  the  greater 
part  of  it  is  included  between  the  periphery  of  the  diaphragm  and  the 
chest-wall. 

Directions. — The  student  should  now  attempt  by  the  following  method 
to  restore  the  lung  as  nearly  as  possible  to  its  natural  dimensions 
and  relations.  The  nozzle  of  a  pair  of  bellows  should  be  wrapped 
firmly  round  with  a  strip  of  wet  cloth  until  it  is  made  of  a  convenient 
size  to  fit  the  trachea,  which  is  to  be  cut  across  about  the  middle  of  the 
neck  for  its  reception.  The  nozzle  is  then  to  be  tied  tightly  into  the 
trachea  with  a  thick  piece  of  string  cai-ried  several  times  round,  and  the 
lung  is  to  be  gradually  inflated  while  an  assistant  guides  it  into  position, 
and  guards  it  from  being  wounded  by  the  cut  ends  of  the  ribs.  Provided 
the  lung  has  not  been  injured,  it  can  by  this  means  be  restored  to  its 
natural  position,  and  the  student  should  then  observe  the  area  of 
pericardium  which  is  left  uncovered  at  the  notch  in  the  -lower  border. 
The  right  side  of  the  chest  may  next  be  opened,  making  the  same 
incisions  as  on  the  left.  On  raising  the  base  of  the  right  lung  from 
the  diaphragm,  its  supernumerary  lobule  will  be  seen,  and  also  the 
posterior  vena  cava  and  right  phrenic  nei-ve  invested  by  the  special  fold 
of  pleura.     The  right  lung  may  then   be  inflated,   and  the  extent  of 


DISSECTION    OF    THE    BACK   AND    THORAX.  105 

pericardium  left  uncovered  by  lung  on  this  side  should  be  observed. 
Thereafter,  the  lung  should  be  reflected  towards  the  spine,  and  the 
pleura  should  be  stripped  off  its  root.  The  vessels,  nerves,  and  bronchus 
should  be  isolated  by  teasing  and  scraping,  rather  than  by  cutting. 

The  Root  of  the  Lung,  it  will  be  observed,  is  placed  behind  the  upper 
part  of  the  heart ;  and  it  is  composed  of  the  bronchus,  bloodvessels, 
lymph-vessels,  and  nerves  of  the  lung,  Avith  some  connective-tissue. 
The  bronchus  enters  each  root  in  front  and  above  ;  the  pulmonary  veins 
enter  behind ;  and  the  pulmonary  artery  enters  in  front  of  the  veins. 

Vessels,  Two  sets  of  vessels  pass  to  and  from  the  lung  at  the  root, 
viz.,  the  pulmonary  artery  and  veins,  and  the  bronchial  artery  and  vein. 

The  Pulmonary  Artery  is  the  enormous  vessel  carrying  impure  (venous) 
blood  from  the  right  ventricle  to  be  purified  in  the  capillaries  on  the 
air-cells  of  the  lung.  It  will  be  recognised  by  the  thickness  of  its  wall. 
The  lyubnonary  veins  bring  the  purified  (arterial)  blood  back  from  the  air- 
cells,  and  discharge  it  into  the  left  auricle.  They  form  at  the  root  of 
the  lung  from  two  to  four  trunks,  which  are  extremely  short,  especially 
on  the  left  side.  The  pulmonary  vessels  are  the  functional  vessels  of 
the  lung. 

The  Bronchial  Artery  is  a  slender  vessel  entering  the  lung  on  the 
bronchus.  It  carries  nutritive  or  pm-e  blood  to  the  lung  structure,  and 
may  therefore  be  termed  the  nutrient  artery  of  the  lung.  The  blood  which 
it  carries  is  led  out  of  the  lung  by  the  hronchial  vein,  which  joins  the 
coronary  sinus  of  the  heart. 

The  Nerves  of  the  lung  are  derived  from  the  vagus,  as  will  be  seen 
at  a  later  stage.  They  form  a  plexus  at  its  root,  and  pass  along  the 
bronchi  into  its  interior. 

Directions. — Both  lungs  are  to  be  left  until  the  heart  and  the  thoracic 
vessels  and  nerves  have  been  examined.  Proceed  now  to  the  examina- 
tion of  the  pericardium.  It  is  best  examined  from  the  left  side,  and 
Avill  be  sufficiently  exposed  by  hooking  the  left  lung  towards  the  spine. 

THE  pericardium  (pLATES  22    AND    26). 

The  pericardium  is  the  bag  that  contains  the  heart.  It  occupies  a 
position  about  the  centre  of  the  thorax,  and  between  the  right  and  left 
layers  of  the  mediastinum.  The  sac  is  fibrous  in  structure,  and  is  lined 
internally  by  a  serous  membrane.  Like  the  organ  which  it  encloses, 
the  pericardium  has  a  conical  form,  the  point  of  the  cone  being  fixed  to 
the  floor  of  the  sternum  from  about  the  third  chondro-sternal  joint  to 
within  an  inch  of  the  insertion  of  the  diaphragm  across  the  ensiform 
cartilage.  Above  the  sac  is  pierced  by  the  large  vessels  of  the  heart,  and 
there  its  fibrous  texture  blends  with  the  outer  coat  of  the  vessels. 
Its  outer  surface  is  overspread  by  the  mediastinal  pleura,  which  can 
easily  be  stripped  oft". 


106  THE  ANATOMY  OF  THE  HOIWE. 

Directions. — The  pericardium  should  be  piuclied  up,  and  slit  from  its 
apex  to  near  the  base  of  the  heai*t. 

The  pericardium  is  considerably  larger  than  the  heart  which  it  con- 
tains, but  this  disposition  is  not  very  evident  until  it  is  opened,  when 
the  sac  can  be  pulled  away  from  the  heart,  and  a  considerable  cavity  left 
between  them.  The  inner  surface  of  the  bag  and  the  outer  surface  of  the 
heart  are  overspread  by  a  serous  covering — the  serous  membrane  of  the 
pericardium.  The  parietal  division  of  this  membrane  is  that  which 
lines  the  sac ;  the  visceral  division  covers  the  heart  and  the  roots  of  the 
great  vessels  at  its  base,  investing  the  aorta  and  pulmonary  artery  in  a 
common  tube.  The  visceral  portion  is  also  known  as  the  epicardium, 
and  around  the  base  of  the  heart  it  is  continuous  with  the  parietal 
division.  The  free  surface  of  this,  as  of  other  serous  membranes,  is 
exquisitely  smooth,  and  is  formed  by  a  single  layer  of  endothelial  cells. 
Its  object  is  to  facilitate  the  movements  of  the  heart  in  its  sac ;  and  for 
this  purpose,  it  is  kept  moist  by  a  miikute  quantity  of  serous  fluid.  As 
with  the  pleura,  the  cavity  of  the  serous  sac  is  only  a  potential  one ;  but 
when  inflammatory  or  other  effusions  are  poured  out  by  the  membrane, 
it  becomes  an  actual  cavity,  and  the  parietal  and  visceral  layers  of  the 
membrane  may  be  pushed  widely  apart.  In  old  and  emaciated  subjects 
this  is  not  infrequently  the  case,  the  cavity  containing  a  considerable 
quantity  of  watery,  dropsical  fluid. 

THE  HEART  (PLATES  23  AND  24). 

Directions. — The  pericardium  may  now  be  slit  transversely,  and  the 
heart  should  be  tilted  out  by  introducing  the  hand  beneath  its  apex. 
This  will  permit  the  examination  of  the  exterior  of  the  heart  without 
destroying  any  of  its  connections ;  and  afterwards,  in  order  to  observe 
accTirately  its  position,  it  should  be  restored  within  the  pericardium. 

Exterior  of  the  Heart,  The  heart  is  a  hollow  muscular  organ,  and 
acts  the  part  of  a  force-pump  in  maintaining  the  circulation  of  the  blood. 
In  its  interior  there  are  four  cavities — two  auricles  and  two  ventricles, 
the  auricle  of  each  side  being  placed  above  the  ventricle.  This  sub- 
division of  the  interior  of  the  heart  into  cavities  is  indicated  on  its 
exterior  by  certain  grooves.  Thus,  the  auriculo-ventriadar  groove  runs 
around  the  heart  like  a  belt,  and  marks  off"  the  auricles  from  the 
ventricles.  Although  this  gi-oove  is  carried  quite  round  the  heart,  it 
is  not  very  evident  in  front,  being  concealed  there  by  the  origins  of  the 
aorta  and  pulmonary  artery.  Two  other  grooves,  one  on  the  right  side, 
the  other  on  the  left,  descend  from  the  base  of  the  heart,  and  become 
continuous  a  little  in  front  of  the  apex.  These  grooves  correspond  to 
the  edges  of  the  septum  which  separates  the  cavities  of  the  right  side 
fi'om  those  of  the  left.  They  are  much  more  distinctly  marked  on  the 
ventricular  portion  of  the  heart,  where  they  are  termed  the  ventricular 


DISSECTION    OF    THE    BACK    AND    THORAX.  107 

grooves.  The  grooves  of  the  heart  lodge  the  coronary  vessels,  and  a 
quantity  of  fat  which  is  present  in  all  but  the  most  emaciated  subjects. 

In  foi-m  tlie  heart  resembles  a  cone  compressed  from  side  to  side ; 
and  its  exterior  may  be  described  as  presenting  two  surfaces,  two 
borders,  a  base,  and  an  apex.  The  surfaces  of  the  heart  are  formed  by 
the  ventricles ;  and  when  these  cavities  are  distended,  both  surfaces  are 
convex.  The  right  side  of  the  heart  is  formed  principally  by  the  right 
ventricle,  but  partly  also  by  the  left.  The  right  ventricular  groove 
descends  on  it,  and  crosses  round  the  anterior  border  a  little  above  the 
apex.  The  left  side  belongs  chiefly  to  the  left  ventricle,  but  partly  in 
front  to  the  right.  It  shows  the  left  ventricular  furrow  crossing 
towards  the  anterior  border,  where  it  joins  the  furrow  of  the  opposite 
side.  The  anterior  border  of  the  heart  is  slightly  convex,  and  has  an 
obliqvie  direction  downwards  and  backwards  when  the  heart  is  in  position. 
It  belongs  nearly  altogether  to  the  right  ventricle,  the  two  ventricular 
furrows,  which  denote  the  position  of  the  septum  between  the  two 
cavities,  becoming  continuous  round  this  border  a  little  above  the  apex. 
The  posterior  border  is  thicker  and  less  flaccid  than  the  anterior.  It  is 
nearly  straight,  and  is  disposed  almost  vertically  when  the  heart  is  in 
position.  It  belongs  entirely  to  the  left  ventricle.  The  base  of  the 
heart  lies  above,  and  is  formed  by  the  auricles.  At  this  point  the 
large  vessels  pass  to  and  from  the  heart,  and  form  the  principal 
means  of  its  suspension.  The  left  auricle  forms  the  left  posterior 
part  of  the  base,  and  consists  of  a  sinus  venosus  into  which  the 
pulmonary  veins  open,  and  an  ear-shaped  appendix — the  auricula,  the 
latter  being  most  posterior.  The  right  auricle  forms  the  right  anterior 
part  of  the  base,  and  also  consists  of  a  sinus  venosus  and  an  auricula, 
the  latter  lying  in  front.  The  large  systemic  veins  discharge  them- 
selves into  the  sinus  venosus  of  the  right  auricle.  The  a2:)€x  of  the 
heart  is  blunt  and  firm,  and  belongs  to  the  left  ventricle. 

Position  and  relations. — In  order  to  study  these,  the  heart  should  now 
be  restored  to  its  natural  position  Avithin  the  pericardium. 

The  position  of  the  heart  may  be  expressed  with  regard  to  the  skeleton 
as  follows  : — It  lies  beneath  the  bodies  of  the  dorsal  vertebrse  from  the 
4th  to  the  10th  inclusive;  it  responds  to  the  four  ribs  behind  the  2nd; 
and  it  is  placed  above  the  sternum  from  about  the  3rd  chondro-sternal 
joint  to  within  an  inch  of  its  posterior  extremity. 

In  a  medium-sized  animal  the  most  anterior  pai-t  of  the  heart  (right 
auricula)  is  distant  about  four  or  five  inches  from  the  entrance  to  the 
chest ;  the  posterior  liorder  at  its  upper  part  is  separated  by  about  the 
same  interval  from  the  tendinous  centre  of  the  diaphragm,  but  at  its 
lower  part  it  is  only  about  an  inch  in  front  of  the  insertion  of  the  rim  of 
the  diaphragm  across  the  ensiform  cartilage ;  and  during  great  distension 
of  the  abdominal  viscera,  the  diaphragm  may  be  driven  forwards  so  as 


108  THE  ANATOMY  OK  THE  HOU8E. 

to  entirely  obliterate  the  interval  between  it  and  the  heart,  a  condition 
which  interferes  not  only  with  respiration  but  also  with  the  movements 
of  the  heart. 

The  base  of  the  heart  has  its  mid  point  a  little  to  the  right  of  the 
mesial  plane  of  the  body,  and  is  distant  about  six  inches  from  the 
spine,  to  which  it  is  suspended  by  the  great  systemic  vessels. 

The  apex  of  the  heai-t  lies  over  the  posterior  extremity  of  the  sternum, 
and  slightly  to  the  left  of  the  mesial  plane. 

THE  NERVES  AND  VESSELS  OF  THE  LEFT  SIDE  OF  THE  THORAX  (PLATE  22). 

Po.siiiou. — It  will  be  most  convenient  to  lower  the  trunk  from  its 
suspended  position,  and  lay  the  thorax  flat  on  a  table,  with  the  left  side 
upwards. 

Directions. — Sever  the  insertion  of  the  scalenus  into  the  1st  rib,  and 
then  remove  that  bone  by  sawing  through  its  lower  extremity  and 
disarticulating  its  costo-vertebral  joints.  In  these  operations  take  care 
not  to  ciit  the  vessels  or  nerves  to  the  inner  side  of  the  rib.  In  order 
to  follow  many  of  the  nerves  and  vessels  of  the  thorax,  but  little  dis- 
section is  necessary,  as  they  show  distinctly  through  the  transparent 
pleura  which  covers  them.  The  phrenic,  pneumogastric,  and  cardiac 
nerves  should  be  found  in  the  anterior  mediastinum,  and  traced  back- 
wards ;  the  sympathetic  chain  will  be  seen  at  the  roof  of  the  cavity, 
extending  under  the  costo-vertebral  articulations. 

The  Left  Phrenic  or  Diaphragmatic  Nerve  is  formed  at  the  root  of  the 
neck  by  the  union  of  three  branches  (Plate  3),  the  smallest  of  which  is  not 
constantly  present.  The  inconstant  branch  is  from  the  inferior  primary 
branch  of  the  5th  cervical  nerve,  the  others  are  furnished  by  the  corre- 
sponding branches  of  the  6th  and  7th  cervical  nerves.  The  nerve,  as 
thus  formed,  enters  the  chest  between  the  first  pair  of  ribs,  passing 
between  the  axillary  artery  and  the  origin  of  its  inferior  cervical  branch. 
Continuing  backwards  between  the  layers  of  the  mediastinum,  it  crosses 
the  common  trunk  of  the  dorsal,  superior  cervical,  and  vertebral  veins, 
and  the  pericardium  at  the  level  of  the  common  aorta.  Behind  the 
heart  it  passes  under  the  root  of  the  lung,  through  the  posterior 
mediastinum,  and  is  distributed  to  the  left  half  of  the  diaphragm 
(muscular  rim  and  pillar),  of  which  it  is  the  motor  nerve. 

The  Left  Pneumogastric,  Vagus,  or  10th  Cranial  Nerve.  At  the 
entrance  to  the  chest  this  nerve  lies  on  the  trachea,  at  the  upper  edge 
of  the  cephalic  trunk,  and  a  little  below  the  sympathetic.  It  crosses 
in  beneath  the  arch  of  the  left  axillary,  in  company  with  a  cardiac  nerve. 
It  is  continued  backwards  across  the  angle  of  separation  between  the 
anterior  and  posterior  aorta; ;  and  crossing  the  root  of  the  latter  vessel, 
it  reaches  the  root  of  the  lung,  where  it  divides.  The  upper  division  is 
continued  backwards  to  fuse  above  the  oesophagus  with  the  con-espouding 


DISSECTION    OF    THE   BACK    AND    THORAX.  109 

branch  from  the  right  vagus,  this  fusion  taking  place  about  the  middle 
of  the  posterior  mediastinum.  The  lower  division  unites  in  the  same 
way  with  a  branch  from  the  nerve  of  the  opposite  side,  the  fusion  taking 
place  on  the  left  bronchus.  The  resulting  nerves  are  termed  the 
styyerior  and  inferior  cesojihageal  nerves,  and  they  are  continued  back- 
wards, the  one  above,  and  the  other  below,  the  gullet,  giving  branches  to 
it  and  accompanying  it  through  the  foramen  sinistrum  of  the  dia- 
phragm. * 

In  this  part  of  its  coixrse  the  vagus  detaches  the  following  branches  : — 

1.  A  Branch  of  Communication  with  the  middle  cervical  ganglion  of  the 
sympathetic  (or  with  the  inferior  ganglion  when  the  middle  is  not 
developed).     It  is  given  off"  within  the  1st  rib. 

2.  The  left  Inferior  {recurrent)  Laryngeal  Nerve. — This  is  detached  at 
the  root  of  the  posterior  aorta ;  and  turning  round  behind  the  vessel  at 
that  point,  it  gains  its  inner  side,  to  be  included  between  the  artery  and 
the  left  bronchus,  where  it  receives  twigs  from  the  cardiac  nerves.  It 
then  passes  forwards  along  the  lower  face  of  the  trachea,  in  company 
with  a  cardiac  nerve ;  and  issuing  from  the  chest,  it  is  continued  up  the 
neck  to  the  larynx.  As  it  is  included  between  the  aorta  and  left 
bronchus,  it  is  related  to  the  bronchial  lymphatic  glands.  Within  the 
tliorax  the  nerve  gives  branches  that  pass  upwards  and  forwards  to  the 
trachea  and  oesophagus.  The  left  recui'rent  is  the  nerve  implicated  in 
"roaring." 

3.  Piilmonary  Branches. — These  form  at  the  root  of  the  lung  a  plexus 
from  which  filaments  are  continued  into  the  lung  along  the  ramifications 
of  the  air  tube. 

Dorsal  Roots  of  the  Brachial  Plexus.  These  are  two  branches  of 
the  1st  and  2nd  dorsal  nerves  respectively.  They  will  be  found  at  the 
upper  part  of  the  1st  and  2nd  intercostal  spaces.  After  giving  branches 
to  the  inferior  cervical  ganglion,  they  turn  round  the  inner  surface  of 
the  1st  rib,  close  to  its  upper  extremity. 

The  Sympathetic  Nerve.  The  cervical  cord  of  the  sympathetic, 
which  in  the  neck  is  fused  with  the  vagus,  separates  from  it  at  the  entrance 
to  the  chest,  and  terminates  in  a  stellate  greyish  ganglion — tlie  middle 
cervical  ganglion. 

The  Middle  Cervical  Ganglion. — This  will  be  found  within  the  1st  rib, 
or  in  front  of  it,  at  the  line  of  contact  of  the  trachea  and  oesophagus. 
A  thick  connecting  branch  continues  it  up  to  another  enlargement — the 
inferior  cervical  ganglion.  The  middle  cervical  ganglion  has  a  branch 
of  communication  witli  the  vagus,  and  gives  off"  two  or  three  cardiac 
nerves. 

*  The  superior  cesopliageal  nerve  is  generally,  if  not  always,  larger  than  the  inferior  ;  and  in  most 
cases  I  have  found  that  the  upper  nerve  is  formed  in  greater  proportion  by  the  left  vagus  than  by 
the  right,  while  the  lower  is  formed  about  equally  from  each.  Chauveau,  on  the  other  hand, 
describes  and  figures  the  upper  nerve  as  being  formed  mainly  by  the  right  vagus,  and  the  lower  by 
the  left  vagus. 


110  THE  ANATOMY  OF  THE  HORSE. 

The  Inferior  Cervical  Ganglion  is  placed  a  little  above  the  preceding, 
to  which  it  is  connected  by  a  shoi-t  thick  nerve.  It  rests  on  the  longus 
colli,  between  the  vertebral  and  superior  cervical  arteries.  It  is  joined 
by  the  vertebral  nerve,  and  by  short  branches  from  the  inferior  primary 
divisions  of  the  8th  cervical  and  first  two  dorsal  nerves.  By  its  posterior 
extremity  it  is  continued  into  the  dorsal  cord  of  the  sympathetic.  It 
gives  off  a  cardiac  filament. 

Cardiac  Nerves  of  the  left  side. — These  nerves,  like  many  others, 
have  a  variable  disposition,  but  the  following  is  what  I  have  found  to 
be  the  most  common  arrangement. 

The  middle  cervical  ganglion  detaches  two  cardiac  nen^es :  (1)  One  of 
these  (which  may  be  double  at  its  origin),  the  smaller  of  the  two,  is 
distributed  to  the  great  arteries  in  the  anterior  mediastinum.  (2)  The 
other  immediately  divides  into  two  branches — a  lower  and  an  upper, 
(a)  The  lower  branch,  joined  by  a  filament  from  the  vagus,  passes 
beneath  the  arch  of  the  left  axillary  in  company  with  the  vagus,  and 
reaching  the  angle  of  bifurcation  of  the  common  aorta,  it  divides,  one 
branch  continuing  backwards  on  the  posterior  aorta  to  dip  down 
between  the  right  and  left  divisions  of  the  pulmonary  artery  and 
gain  the  left  auricle,  while  the  other  descends  along  the  common 
aorta,  and  uniting  at  the  origin  of  the  right  coronary  artery  with 
a  cardiac  branch  of  the  right  side,  is  distributed  to  the  roots  of  the 
great  arteries  and  to  the  ventricles,  the  largest  branches  following 
the  right  coronary  artery.  (6)  The  upper  division  of  the  second  nerve, 
passing  to  the  inner  side  of  the  left  axillary,  and  along  the  lower  face 
of  the  trachea,  unites  with  a  right  cardiac  filament,  crosses  to  the 
right  of  the  common  aorta,  and  is  reflected  round  that  trunk  to  gain 
the  left  coronary  artery,  its  divisions  following  the  main  branches  of 
that  vessel. 

The  inferior  cervical  ganglion  detaches  a  slender  cardiac  nerve  which, 
after  throwing  off  some  twigs  to  the  aileries  in  the  anterior  medias- 
tinum, passes  downwards  and  backwards  to  the  left  auricle. 

["  The  cardiac  nerves  of  the  horse  (left  side)  ordinarily  have  the  following  disposition  : 
There  are  found  four  nerves,  two  of  which,  very  slender,  jjroceed  from  the  middle 
cervical  ganglion  and  lose  themselves  on  the  vessels  arising  from  the  convexity  of  the 
brachial  trunk.  The  two  others  are  the  one  superficial,  the  other  deep.  The  superficial 
nerve,  the  more  voluminous,  commences  by  a  filament  which  springs  from  the  middle 
cervical  ganglion,  passes  backward  and  downwards,  contracts  beneath  the  brachial  trunk 
an  anastomosis  en  arcade  witli  a  branch  detached  from  tlie  inferior  cervical  ganglion,  and 
then  places  itself  alongside  of  the  following.  The  deep  nerve  is  formed  at  first  by  three 
elements  :  (1)  of  medullary  fibres  furnished  by  the  spinal  pairs ;  (2)  of  a  ramuscule 
furnished  by  the  cervical  cord  of  the  sympathetic  ;  (3)  of  a  slender  filament  which  pro- 
ceeds from  the  left  pneumogastric  at  the  entrance  to  the  chest.  It  places  itself  in  the 
direction  of  the  heart,  adheres  to  the  superficial  nerve,  is  inflected  on  tlie  concavity  of 
the  brachial  artery,  margins  this  vessel  to  the  left  and  insinuates  itself  between  the  aorta 
and  the  pulmonary  artery.  At  this  point  these  nerves  are  distributed  to  the  heart  and  to 
the  great  vessels,  a  branch  passes  iinder  the  right  auricle  and  plunges  into  the  cardiac 


DISSECTION    OF    THE    BACK    AND    THORAX.  Ill 

muscle ;  a  second  spreads  itself  over  the  origin  of  the  pulmonary  artery  and  over  the 
right  ventricle  ;  two  other  branches,  grayish,  plexiform,  anastomose  more  or  less  between 
the  aorta  and  the  pulmonary  artery,  unite  under  the  aortic  root  with  a  nerve  which 
comes  from  the  right  side,  then  descend  in  the  vertical  furrow  of  the  heart,  and  are 
expended  in  the  left  ventricle  ;  finally,  some  other  ramuscules,  parallel  to  the  pneumo- 
gastric,  are  expended  on  the  pulmonary  artery  and  on  the  aorta." — Ghanrmu.  "  Traite 
d'anatomie  comparee  des  animaux  domestiques. "] 

The  left  Dorsal  Cord  of  the  Syn^xithetic. — This  will  be  seen  through  the 
transparent  pleura,  extending  beneath  the  costo-vertebral  articulations. 
The  first  portion  of  the  cord  is  concealed  at  the  outer  edge  of  the  longus 
colli  muscle,  where  it  joins  the  inferior  cervical  ganglion.  It  crosses 
the  intercostal  vessels  superficially ;  and  in  company  Avith  it,  from  the 
6th  intercostal  space  backwards,  is  the  great  splanchnic  nerve.  Poster- 
iorly it  passes  between  the  psoas  parvus  and  the  left  cms  of  the 
diaphragm,  and  is  continued  as  the  lumbar  cord.  The  cord  is  studded 
with  ganglia  of  a  flattened  form  and  greyish  colour,  there  being  a  gang- 
lion for  each  intercostal  space.  Each  ganglion  is  placed  at  the  posterior 
part  of  the  space  to  which  it  belongs,  and  partly  on  the  posterior  rib. 
It  is  connected  by  an  afferent  filament  to  the  intercostal  nerve  of  the 
same  space,  and  from  it  proceed  other  branches,  Avhich  are  sometimes 
named  efferent.  The  efferent  branches  from  the  first  five  or  six  ganglia 
are  very  small,  and  pass  to  the  adjacent  arteries,  ligaments,  or  vertebrae. 
The  efferent  branches  from  the  succeeding  ganglia  unite  to  form  the 
splanchnic  nerves. 

The  Great  Splanclmic  Nerve  lies  to  the  inner  side  of  the  gangliated 
cord,  as  far  as  the  15th  intercostal  space.  There  it  crosses  to  the  outer 
side,  and  is  continued  backwards  to  enter  the  abdomen  by  passing 
between  the  psoas  parvus  and  the  rim  of  the  diaphragm.  In  the 
abdomen  it  joins  the  semilunar  ganglion.  The  first  efferent  filament 
contributing  to  the  formation  of  the  nerve  comes  usually  from  the  6th 
ganglion,  and  the  last  from  the  16th.  The  intermediate  ganglia  con- 
tribute irregularly,  some  sending  no  branch,  in  which  case  the  next 
ganglion  contributing  sends  a  branch  of  more  than  the  usual  size. 

The  Small  Splanchnic  Nerve  is  either  the  eflFerent  filament  from  the 
17th  ganglion,  or  it  is  formed  by  the  union  of  that  and  the  filament 
from  the  16th.  It  passes  directly  to  the  solar,  the  renal,  or  the  supra- 
renal plexus. 

The  Pulmonary  Artery.  This  is  a  short  vessel  of  enormous  calibre. 
It  springs  from  the  conus  arteriosus  of  the  right  ventricle  ;  and  passing 
in  front  of  the  common  aorta,  it  gains  its  left  side,  crosses  the  root  of 
the  posterior  aorta,  and  divides  behind  it  into  a  right  and  a  left  branch, 
one  for  each  lung.  Each  of  these  enters  the  root  of  the  lung  and 
divides.  As  the  trunk  of  the  pulmonary  artery  rests  on  the  root  of  the 
posterior  aorta,  it  is  connected  to  it,  in  the  adult,  by  a  fibrous  cord 
which  is  the  remains  of  the  ductus  arteriosus— o.  vessel  which  in  the 


112  THE  ANATOMY  OF  THE  HORSE. 

fa;t\is  brings  the  two  arteries  into  comnmnication.  The  puhiionary 
artery  conveys  venous  blood  to  the  lungs  to  be  purified. 

The  Common  Aorta.  This  is  the  primary  trunk  of  the  syxtemic 
arteries.  It  is  of  great  calibre,  but  not  more  than  three  inches  in  length. 
It  springs  from  the  left  ventricle,  and  divides  into  two  unequal  vessels — 
the  anterior  and  the  posterior  aorta.  Whei-e  the  vessel  springs  from 
the  ventricle,  it  shows,  when  injected,  three  bulgings,  each  corresponding 
to  a  dnus  of  Valsalva.  From  two  of  these  sinuses  spring  the  right  and 
left  coronary  arteries  of  the  heart.  These,  wliich  are  the  first  collateral 
branches  of  the  arterial  tree,  are  described  with  the  heart. 

The  Posterior  Aorta  is  by  far  the  longer  of  the  two  terminal  branches 
of  the  common  trunk,  and  it  has  also  the  greater  calibre.  It  passes 
backwai'ds  and  upwards,  describing  a  curve — the  arch  of  the  aorta,  and 
reaches  the  spine  at  the  10th  dorsal  vertebra.  From  that  point  it  is 
continued  backwards  along  the  vertebral  bodies,  being  at  first  a  little  to 
the  left  of  the  middle  line ;  but  it  gradually  inclines  to  the  right,  imtil, 
at  the  14th  dorsal  vertebra,  it  lies  almost  entirely  to  the  right  of  the 
median  plane  of  the  body.  It  passes  into  the  abdominal  cavity  through 
the  hiatus  aorticus — an  opening  between  the  pillars  of  the  diaphragm. 
The  arch  of  the  vessel  is  crossed  to  the  right  by  the  oesophagus,  and  by 
the  termination  of  the  trachea.  The  remaining  portion  of  the  artery 
is  related  on  its  right  to  the  thoracic  duct  and  vena  azygos,  the  duct 
being  usually  between  the  vein  and  artery,  but  sometimes  to  the  left  of 
the  latter.     The  thoracic  branches  of  the  posterior  aorta  are  : — 

1.  The  Broncho-desojyhageal  Artery. — This  vessel  will  be  more  con- 
veniently dissected  with  the  right  side  of  the  chest.  It  is  described  at 
page  118. 

2.  Intercostal  Arteries. — The  last  thirteen  of  these  generally  have  this 
origin.  They  spring  from  the  upper  aspect  of  the  artery,  and  pass  over  the 
vertebral  bodies,  crossing  beneath  the  dorsal  cord  of  the  sympathetic  to 
gain  the  upper  end  of  an  intercostal  space.  Here  each  gives  off  a  large 
dorso-spinal  branch,  and  places  itself  at  the  posterior  border  of  a  rib, 
along  which  it  descends.  The  latter  part  of  the  intercostals  and  their 
dorso-spinal  branches  have  already  been  followed  in  the  dissection  of  the 
chest-wall  and  back. 

The  Anterior  Aorta.  This  vessel,  after  a  course  of  not  more  than 
three  inches,  divides  into  two  vessels  of  unequal  size.  The  left  and  smaller 
of  the  two  is  the  left  axillary  artery ;  the  other  is  the  arteria  innomin- 
ata.  The  direction  of  the  anterior  aorta  is  oblique  upwards  and  for- 
wards, and  it  is  in  great  pai-t  included  within  the  pericardial  sac.  It  has 
no  collateral  branches  of  a  size  meriting  description.  Of  its  terminal 
branches  only  the  left  axillary  will  be  followed  now  The  left  axillary 
is  the  vessel  for  the  supply  of  the  neck,  the  fore  limb,  and  the  subjacent 
part  of  the  chest-wall  on  the  left  side ;  while  the  arteries  innominata, 


DISSECTION    OF    THE    BACK    AND    THORAX.  113 

besides  supplying  the  corresponding  parts  on  the  opposite  side,  carries 
blood  for  the  head. 

The  Left  Axillary  Artery  is  smaller  than  the  other  division  of  the 
anterior  aorta,  and  placed  at  a  higher  level.  It  passes  forwards  in  the 
anterior  mediastinum,  describing  a  curve  which  has  its  convexity 
directed  upwards  and  forwards.  It  leaves  the  chest  by  passing  to  the 
inner  side  of  the  1st  rib,  and  turns  round  the  anterior  border  of  the 
bone,  where  it  leaves  a  smooth  impression  below  the  lowest  fibres  of  the 
scalenus.  From  this  point  it  is  directed  downwards  and  backwards 
across  the  inner  aspect  of  the  shoulder,  beyond  which  it  is  continued  as 
the  brachial  or  humeral  artery.  The  vessel  has  thus  a  part  within  the 
thorax,  and  another  in  the  axilla;  but  only  the  former  presents  itself 
now.  In  the  human  subject  the  artery  passes  beneath  the  clavicle  and 
is  termed  the  Subclavian.  The  arch  formed  by  the  thoracic  part  of  the 
left  axillary  rests  to  its  right  on  the  trachea,  and  touches  at  its  highest 
point  the  oesophagus.  Beneath  the  arch  the  vagus  and  phrenic  nerves, 
and  one  of  the  left  cardiac  nerves  pass  backwards.  The  common  trunk 
of  the  left  dorsal,  superior  cervical,  and  vertebral  veins  crosses  it  on  the 
left,  in  passing  down  to  the  anterior  vena  cava.  The  collateral  branches 
of  the  artery  arising  within  the  chest  are  four  in  number,  three  of  them, 
viz.,  the  dorsal,  superior  cervical,  and  vertebral  ai-teries,  arise  from  the 
summit  of  the  arch  ;  the  other,  the  internal  thoracic  or  mammary  artery, 
takes  origin  from  its  lower  aspect,  at  the  hinder  edge  of  the  1st  rib. 

The  Dorsal  Artery  passes  upwards  and  forwards  across  the  oesophagus 
and  longus  colli  muscle,  and  disappears  at  the  upper  end  of  the  2nd 
intercostal  space.  At  the  outer  edge  of  the  longus  colli  it  gives  off  the 
subcostal  artery,  a  vessel  which  furnishes  the  2nd,  3rd,  and  4th  intercostal 
arteries.  The  2nd  intercostal  artery  may  arise  directly  from  the  trunk 
of  the  dorsal.  External  to  the  chest  the  dorsal  artery  is  distributed  to 
the  parts  beneath  the  scapula,  and  to  the  upper  part  of  the  neck. 

The  Superior  Cervical  Artery  arises  a  little  in  advance  of  the  pre- 
ceding. It  crosses  the  oesophagus  and  longus  colli,  and  perforates  the 
upper  end  of  the  1st  intercostal  space.  It  supplies  the  1st  intercostal 
artery ;  and,  external  to  the  chest,  it  is  distributed  in  the  neck. 

The  Vertebral  Artery  has  its  origin  a  little  in  front  of  the  preceding. 
Passing  obliquely  upwards  and  forwards,  it  crosses  the  inner  side  of  the 
1st  rib  near  its  upper  extremity,  and  enters  the  root  of  the  neck  to  pass 
iu  succession  through  the  series  of  vertebral  foramina. 

The  Internal  Thoracic  Artery  arises  at  the  inner  side  or  hinder 
edge  of  the  1st  rib.  It  descends  along  the  inner  face  of  the  rib  to  the 
floor  of  the  chest,  where  it  will  be  followed  at  a  later  stage. 

The  Superior  Cervical  and  External  Thoracic  Arteries  are  given 
off  from  the  axillary  at  the  anterior  edge  of  the  1st  rib,  and  their  roots 
may  be  seen  now,  but  they  are  distributed  to  parts  without  the  thorax. 

I 


114  THE  ANATOMY  OF  THE  HORSE, 

The  Anterior  Vena  Cava.  This  large  vessel  will  be  seen  below  the 
large  arteries  in  the  anterior  mediastinum.  It  is  formed  at  the  entrance 
to  the  chest  by  the  union  of  the  axillary  and  jugular  veins  of  both 
sides,  and  it  terminates  in  the  i-ight  auricle.  It  is  better  seen  on  the 
right  side  of  the  thorax,  and  will  be  more  fully  described  in  that 
connection.     It  receives  the  following  branches  on  this  side  : — 

1.  The  Internal  Thoracic,  wliich  accompanies,  and  exactly  con-esponds 
to,  the  homonymous  artery. 

2.  A  lai'ge  venous  trunk  formed  by  the  union  of  the  vertebral,  superior 
cervical,  and  dorsal  veins.  It  crosses  to  the  left  of  the  axillary  artery  to 
reach  the  vena  cava.  Very  exceptionally,  as  in  Plate  23,  the  vertebral 
vein  may  join  the  cava  independently.  The  vertebral  and  superior 
cervical  veins  exactly  correspond  to  the  arteries  of  the  same  names,  but 
the  subcostal  root  of  the  dorsal  vein  is  of  greater  extent  than  the  corre- 
sponding artery,  for  it  drains  the  intercostal  spaces  from  the  3rd  to  the 
11th  or  12th.  The  left  dorsal  vein  is  also  called  the  small  ve7ia  azygos. 
These  veins  are  supei'ficially  placed  to  the  corresponding  arteries  as 
they  lie  on  the  oesophagus  and  longus  colli. 

Intercostal  Veins. — The  last  five  or  six  of  the  left  side  join  the  great 
vena  azygos,  a  vessel  of  the  right  side  of  the  chest. 

The  Thoracic  Duct.  This  is  the  largest  lymphatic  vessel  in  the 
body,  and  has  a  calibre  about  twice  that  of  a  goose  quill.  It  will  be 
most  readily  found  in  the  angle  of  separation  of  the  anterior  and 
posterior  aortse,  resting  on  the  trachea,  at  the  lower  edge  of  the 
oesophagus.  It  will  be  recognised  as  a  very  thin-walled  vessel,  empty 
or  with  a  small  amount  of  coloured  contents,  so  that  it  might  be  mis- 
taken for  a  vein.  There  is  not,  however,  any  vein  of  so  large  a  size  in 
this  situation.  Open  it,  and  pass  a  blunt  probe  along  it  towards  the 
entrance  of  the  chest.  Most  commonly  the  duct  has  the  following 
course  : — Entering  the  chest  by  the  hiatus  aorticus,  to  the  right  of  the 
posterior  aorta,  it  extends  forwards  along  the  spine,  having  the  aorta  on 
its  left  and  the  great  vena  azygos  on  its  right.  It  descends  from  the 
spine  on  the  right  side  of  the  aortic  arch,  crosses  the  before-mentioned 
angle,  where  it  rests  on  the  trachea.  It  then  passes  to  the  right  side  of 
the  left  axillary  artery,  and  dips  down  between  that  vessel  and  the 
arteria  innominata.  It  terminates  at  the  anterior  edge  of  the  1st  rib, 
in  the  angle  of  junction  of  the  left  jugular  with  its  fellow  or  with  the 
left  axillary  vein,  that  is,  at  the  beginning  of  the  anterior  vena  cava. 
At  its  termination  it  is  slightly  dilated,  and  furnished  with  a  valve. 
The  duct  may  be  found  to  the  left  of  the  posterior  aorta,  or  it  may  be 
double  as  far  as  the  heart,  there  being  a  branch  on  each  side  of  the 
aorta.  The  thoracic  duct  discharges  into  the  venous  system  the 
lymph  collected  throughout  the  whole  animal  except  the  right  fore 
limb,  and  the  right  side  of  the  head,  neck,  chest-wall,  and  diaphragm. 


DISSECTION   OF   THE   BACK   AND    THORAX.  115 

It  is  by  this  channel,  also,  that  the  chyle  absorbed  from  the  intestine 
enters  the  red-blood  vessels.  The  before-mentioned  exceptional  areas 
are  drained  by  the  right  lymphatic  duct,  a  short  vessel  to  be  sought 
afterwards  on  the  right  side. 

The  Trachea.  The  thoracic  portion  of  the  windpipe  is  situated  in 
the  middle  plane  of  the  cavity.  Entering  between  the  first  pair  of  ribs, 
it  passes  backwards  through  the  anterior  mediastinum  ;  and  over  the 
base  of  the  heart  it  bifurcates  to  form  the  right  and  left  bronchi.  The 
angle  of  bifurcation  is  under  the  6th  dorsal  vertebra.  It  is  related  above 
to  the  oesophagus  and  right  longus  colli ;  and  below  to  the  cephalic 
trimk,  arteria  innominata,  anterior  vena  cava,  and  right  auricle.  On  its 
left  side  are  the  arch  of  the  axillary,  the  thoracic  duct,  and  the  arch  of 
the  posterior  aorta.  On  the  right  side  it  is  crossed  near  its  termination 
by  the  great  vena  azygos,  as  will  be  seen  at  a  later  stage.  It  is  also 
related  to  the  vagus,  recurrent,  sympathetic,  and  cardiac  nerves ;  and  to 
the  prepectoral,  tracheal,  and  bronchial  lymphatic  glands. 

The  ffisoPHAGUS.  At  the  entrance  to  the  chest  the  gullet  lies  above 
the  trachea,  and  a  little  to  its  left  side ;  but  as  it  passes  backwards 
beneath  the  longus  colli  muscle,  it  mounts  on  to  the  middle  of  the 
upper  face  of  the  trachea,  and  passes  directly  over  its  bifurcation, 
having  the  arch  of  the  aorta  to  its  left.  Beyond  that  it  enters  the 
posterior  mediastinum,  between  whose  layers  it  passes,  a  few  inches 
below  the  spine,  to  perforate  the  diaphragm  by  the  foramen  sinistrum. 
At  its  entrance  into  the  chest,  and  for  some  distance  beyond  that  point, 
the  muscular  wall  of  the  tube  is  red,  but  behind  the  heart  it  is  pale. 

The  Structure  of  the  oesophagus  is  described  at  page  150,- that  of  the 
trachea  at  page  149. 

Lymphatic  Glands  of  the  thorax.  The  following  groups  of  glands 
may  be  seen  at  this  stage  : — 

1.  (Esophageal  glands  of  small  size,  along  the  oesophagus,  between  the 
layers  of  the  posterior  mediastinum. 

2.  Bronchial  glands,  situated  at  the  bifurcation  of  the  trachea,  and 
extending  along  the  bronchi.  The  lymphatic  vessels  of  the  lung  join 
these. 

3.  Tracheal  or  Cardiac  glands,  a  double  chain  of  glands  on  the  lower 
face  of  the  trachea,  in  the  anterior  mediastinum,  and  placed  on  the 
course  of  the  lymphatic  vessels  from  the  heart. 

4.  A  series  of  small  glands  beneath  the  pleura,  at  the  upper 
•extremities  of  the  intercostal  spaces. 

5.  Prepectoral  glands. — These  belong  to  the  neck  rather  than  to  the 
thorax,  but  some  of  them  may  have  been  left  by  the  dissector  of  the 
former  region.  They  are  situated  at  the  entrance  to  the  chest,  beneath 
the  great  vessels. 

The   Thymus  Gland.     In  the  foetus  this  is  a   considerable   organ. 


116  THE  ANATOMY  OF  THE  HORSE. 

composed  of  lymphoid  tissue,  and  included  between  the  layers  of  the 
anterior  mediastinum.  It  steadily  atrophies  after  birth,  and  in  the 
adult  only  the  shrivelled  remains  of  it  will  be  found. 

THE   NERVES    AND    VESSELS   OF   THE   RIGHT   SIDE   OF   THE   THORAX 
(plates  25  AND  26). 

Directions. — Reverse  the  position  of  the  thorax,  turning  the  right 
side  upwards,  and  proceed  as  already  directed  for  the  display  of  the 
structures  on  the  left  side  (page  108). 

The  Right  Phrenic  Nerve.  This  nerve  enters  the  chest  by  passing 
beneath  the  right  axillary  artery,  being  included  between  that  vessel 
and  the  anterior  vena  cava.  In  the  anterior  mediastinum  it  lies  on  the 
side  of  the  anterior  vena  cava.  It  crosses  the  pericardium  as  on  the  left 
side,  and  behind  the  heart  it  passes  across  or  below  the  posterior  vena 
cava  to  reach  the  diaphragm,  where  it  terminates.  Behind  the  heai't  the 
nerve  and  the  vena  cava  are  included  between  the  layers  of  a  special 
fold  of  pleura  which  passes  upwards  from  the  diaphragm  and  floor  of  the 
chest  to  envelop  them. 

The  Right  Vagus.  Separating  from  the  cervical  cord  of  the 
sympathetic,  the  right  vagus  enters  the  chest  by  passing  under  the 
arch  of  the  right  axillary  in  company  with  a  cai'diac  nerve,  having 
the  anterior  vena  cava  below.  It  is  then  directed  obliquely  back- 
wards and  upwards  across  the  trachea;  and  crossing  to  the  inner 
side  of  the  great  vena  azygos,  it  divides  at  the  line  of  contact  of  the 
gullet  and  windpipe.  Each  branch  unites,  as  ah*eady  described, 
with  the  con-esponding  branch  of  the  left  vagus,  thus  forming  the 
superior  and  inferior  oesophageal  nerves.  The  thoracic  bi'anches  of 
the  right  vagus  are  : — 

1.  Branches  of  Communication  Avith  the  middle  and  inferior  cervical 
ganglia  of  the  sympathetic. 

2.  The  right  Inferior  (recurrent)  Laryngeal. — This  nerve  differs  from 
the  left  in  its  relations  and  point  of  origin.  It  is  given  off  from  the 
vagus  at  the  origin  of  the  dorso-cervical  artery.  Turning  round  behind 
the  root  of  this  trunk,  between  it  and  the  trachea,  it  passes  forwards 
on  the  lower  face  of  the  windpipe,  above  the  cephalic  artery,  and  internal 
to  the  middle  cervical  ganglion  of  the  sympathetic.  Reaching  the  root 
of  the  neck,  it  crosses  between  the  carotid  artery  and  the  trachea,  and 
is  continued  up  the  neck  below  the  artery.  In  the  larynx  it  is  distri- 
buted in  the  same  manner  as  the  left.  In  the  chest  it  communicates 
with  the  cardiac  nerves  and  with  the  middle  cervical  ganglion  of  the 
sympathetic,  and  emits  tracheal  and  oesophageal  filaments  as  on  the  left 
side.  The  right  recurrent  nerve,  it  will  be  observed,  is  considerably 
shorter  than  the  left,  having  its  origin  at  the  posterior  edge  of  the  1st 
rib,  while  the  left  has  its  origin  at  the  base  of  the  heart.     Moreover,  the 


DISSECTION    OF    THE    BACK    AND    THORAX.  117 

right  is  reflected  round  a  comparatively  small  artery,  while  the  left  is 
reflected  round  the  great  aorta  The  right  recurrent  nerve  is  not  impli- 
cated in  "roaring." 

3.  Cardiac  Bramhes,  variable  in  number,  pass  downwards  and  back- 
wards to  reach  the  lower  face  of  the  trachea,  whence,  after  anastomosing 
intricately  with  the  sympathetic  cardiac  nerves,  they  pass  on  to  the 
right  auricle. 

4.  Pulmonary  Branches  as  on  the  left  side. 

Dorsal  Roots  of  the  Brachial  Plexus.  These  do  not  differ  from 
those  of  the  left  side  (page  109). 

The  Sympathetic  Nerve. 

The  Middle  Cervical  Ganglion. — This  resembles  that  of  the  left  side. 
It  is  placed  on  the  trachea,  internal  to  the  insertion  of  the  scalenus 
muscle  into  the  1st  rib.  It  receives  the  cervical  cord  of  the  sym- 
pathetic in  front,  and  behind  it  is  prolonged  by  a  short  cord  connecting 
it  to  the  inferior  cervical  ganglion.  It  communicates  with  the  vagus 
and  recuiTent  nerves,  and  gives  off  the  cardiac  nerve  accompanying  the 
vagus  beneath  the  axillary  artery. 

The  Inferior  Cervical  Ganglion  is  sitiiated  on  the  longus  colli,  at  the 
upper  edge  of  the  trachea,  and  between  the  vertebral  and  superior  cervi- 
cal arteries.  It  receives  the  vertebral  nerve  and  branches  from  the 
inferior  primary  divisions  of  the  8th  cervical  and  first  two  dorsal  nerves, 
and  is  continued  into  the  dorsal  cord  of  the  sympathetic  as  on  the  left 
side.     It  emits  a  cardiac  nerve. 

Cardiac  Nerves  of  the  right  side. — (1)  The  middle  cervical  ganglion 
detaches  a  considerable  cardiac  nerve  which  accompanies  the  right 
vagus  in  passing  back  beneath  the  arch  of  the  axillary  artery.  Reach- 
ing the  lower  face  of  the  trachea,  it  vmites  with  one  of  the  cardiac 
nerves  of  the  left  side,  and  is  reflected  behind  the  common  aorta  to  be 
distributed  to  the  left  side  of  the  heart,  as  already  described.  This 
nerve  emits  a  branch  to  unite  with  another  cardiac  nerve  of  the  left 
side — that  which  follows  the  right  coronary  artery.  (2)  The  inferior 
cervical  ganglion  gives  origin  to  a  cardiac  nerve,  smaller  than  the 
preceding,  which  it  joins  after  giving  fibres  to  the  right  vagus  and 
recurrent.  (3)  The  cardiac  branches  of  the  right  vagus  have  already 
been  seen. 

[' '  On  the  right  side  we  reckon  two  principal  cardiac  nerves  and  four  secondary  filaments. 
The  first  cardiac  nerve  is  a  long  branch  which  takes  origin  at  the  level  of  the  middle  cer- 
vical ganglion.  It  is  formed  by  fibres  from  the  sympathetic  and  by  a  fasciculus  furnished 
by  the  right  pneumogastric,  at  the  entrance  to  the  chest ;  it  receives  probably  also  some 
medullary  fibres  through  the  intermediation  of  a  branch  of  communication  thrown 
between  the  middle  ganglion  and  the  inferior  ganglion.  This  nerve  is  reinforced  by  two 
filaments  which  proceed  from  the  inferior  cervical  ganglion,  and  sometimes  from  the 
second  middle  ganglion,  of  which  one,  the  posterior,  is  reinforced  in  the  same  way  by  a 
left  sympathetic  filament  which  gains  its  destination  in  passing  alongside  of  the  recurrent 


118  THE  ANATOMY  OF  THE  HORSE. 

nerve.  "WTien  it  is  fully  constituted  the  first  right  nerve  creeps  over  the  base  of  the 
heart,  turns  round  the  root  of  the  aorta,  and  mixes  its  terminal  filaments  with  those  of 
the  left  cardiac  nerves.  The  second  right  cardiac  nerve  is  formed  by  the  union  of  three 
branches  which  take  origin  in  succession  from  the  corresponding  pneumogastric,  behind 
the  dorsal  artery,  along  tlie  right  side  of  the  trachea.  This  nerve  is  in  communication 
with  the  sympathetic  of  the  dorsal  region  by  three  branches  which  approach  the  last 
beneath  the  first,  fourth,  and  sixth  ribs. 

^^^len  the  second  right  nerve  arrives  above  the  termination  of  the  anterior  vena  cava, 
it  divides  into  two  branches  :  the  one  is  thrown  into  the  roof  of  the  auricles  ;  the  other, 
reinforced  by  a  filament  coming  from  the  pneumogastric,  is  expended,  by  numerous 
filaments,  on  the  surface  of  the  left  ventricle  ;  some  reaching  as  far  as  the  right  ventricle. 

The  foiu-  secondary  filaments  are  arranged  like  the  steps  of  a  ladder  on  the  portion  of 
the  pneumogastric  comi)rised  between  the  entrance  of  the  chest  and  the  division  of  the 
bronchi.  These  filaments  are  expended  in  the  great  vessels  and  in  the  walls  of  the  heart." 
— Chauveau.     "  Traits  d'anatomie  comparee  des  animaux  domestiques. "] 

The  Right  Dorsal  Cord  of  the  Sympathetic  does  not  differ  materially 
from  the  left. 

The  Posterior  Aorta  is  here  seen  in  a  large  part  of  its  com-se,  but  it  has 
already  been  fully  described  in  connection  with  the  left  side.  It  detaches 
to  this  side  thirteen  intercostals,  exactly  similar  to  those  of  the  left. 

The  Broxcho-CEsophageal  Artery.  This  artery  arises  from  the 
convexity  of  the  aortic  arch,  a  little  anterior  to  the  bifurcation  of  the 
trachea.  Generally,  as  in  Plate  24,  it  ai-ises  not  independently  but  as 
a  division  of  a  short  vessel  which  is  at  the  same  time  the  common 
trunk  for  the  1st  and  2nd  pairs  of  aortic  intercostals.  It  is  a  small 
vessel,  not  larger  than  an  intercostal.  It  is  reflected  downwards  and 
backwards  on  the  right  side  of  the  aorta,  and  divides  into  the  bronchial 
trunk  and  the  oesophageal  artery.  The  (esophageal  artery,  which  is  the 
smaller  of  the  two,  is  continued  backwards  above  the  gullet,  through  the 
posterior  mediastinum,  extending  sometimes  to  near  the  foramen  sinis- 
trum,  and  anastomosing  with  the  pleuro-ocsophageal  branch  of  the 
gastric  artery.  Sometimes  there  is  an  analogous  vessel  in  the  mediasti- 
num below  the  oesophagus  (inferior  oesophageal),  but  when  present  this  is 
a  very  slender  artery.  The  bronchial  trunk  dips  down  between  the 
aorta  and  the  gullet,  and  bifurcates  to  form  the  right  and  left  bronchial 
arteries,  each  of  which  enters  the  root  of  the  lung  on  the  bronchus.  It 
is  the  nutrient  vessel  to  the  lung.  The  above-mentioned  inferior  oesopha- 
geal may  be  a  branch  of  one  of  the  bronchial  arteries. 

The  Arteria  Innominata  is  the  right  division  of  the  anterior  aorta, 
the  left  axillary  artery  being  the  other  division.  In  calibre  it  is  greater 
than  the  left  axillary,  and  it  is  placed  on  a  lower  level.  It  is  related  to 
the  trachea  above ;  and  to  the  anterior  vena  cava  below  and  to  the  left. 
After  a  course  of  about  two  inches,  it  divides  to  form  the  cephalic  trunk 
and  the  right  axillary  artery,  and  immediately  in  front  of  its  point  of 
division  it  detaches  the  dorso-cervical  artery. 

The  Dorso-cervical  Artery.  This  is  a  short  trunk  which  passes 
upwards  on  the  trachea,  and  divides  to  form  the  dorsal  and  superior 


DISSECTION    OF    THE  BACK   AND   THORAX.  119 

cervical  arteries,  which  have  precisely  the  same  course  and  distribution 
as  those  of  the  left  side.  They  have  also  the  skme  connections,  save  that 
they  do  not  touch  the  oesophagus. 

The  Cephalic  Artery.  This  vessel,  which  has  a  length  of  about  two 
or  three  inches,  passes  directly  forwards  at  the  lower  face  of  the  trachea, 
and  bifurcates  at  the  entrance  to  the  chest,  forming  the  common  carotid 
arteries  (right  and  left). 

The  Right  Axillary  Artery.  This  vessel  in  its  intrathoracic  course 
forms  a  continuous  curve,  or  arch,  with  the  arteria  innominata;  this 
arch  being,  however,  on  a  lower  level,  and  less  abrupt,  than  that  of  the 
left  axillary.    It  gives  off  here  the  vertebral  and  internal  thoracic  arteries. 

The  Vertebral  Artery  and  the  Internal  Thoracic  Artery  do  not 
differ  from  the  homonymous  vessels  of  the  left  side. 

The  Anterior  Vena  Cava.  This  large  vessel,  already  referred  to,  is 
best  seen  from  the  right  side  of  the  chest.  It  is  formed  at  the  entrance 
to  the  chest  by  the  union  of  the  jugular  and  axillary  veins  of  both  sides, 
and  its  initial  portion  is  fixed  by  fibrous  processes  to  the  inner  surfaces 
of  the  first  pair  of  ribs.  It  passes  backwards  through  the  anterior  medias- 
tinum, being  there  related  to  the  great  arteries,  beneath  and  to  the  right 
of  which  it  is  placed.*  It  enters  the  roof  of  the  right  auricle.  Besides  the 
vessels  already  seen  entering  it  on  the  left  side  (page  14),  it  receives — 

1.  The  Internal  Thoracic  Vein. 

2.  The  Vertebral  Vein. 

3.  A  trunk  formed  by  the  union  of  the  dorsal  and  sujoerior  cervical 
veins.     (These  veins  may  enter  independently.) 

4.  The  G7-eat  Vena  Azygos  (sometimes).  This  large  vein  begins 
behind  the  hiatus  aorticus,  where  it  receives  the  first  pair  of  lumbar 
veins.  Passing  through  the  hiatus,  it  extends  along  the  dorsal  portion 
of  the  spine  to  the  right  of  the  posterior  aorta,  the  thoracic  duct  being 
usually  between  the  two  vessels.  At  the  6th  or  7th  dorsal  vertebra 
it  leaves  the  spine  and  curves  downwards  to  the  right  of  the  aortic  arch, 
the  oesophagus,  and  the  trachea;  terminating  either  in  the  anterior  vena 
cava,  or  in  the  auricle  immediately  behind  the  opening  of  that  vein. 
Besides  the  first  pair  of  lumbar  veins,  it  receives  the  last  thirteen 
intercostal  veins  of  the  right  side  and  the  last  five  or  six  of  the  left. 

The  Posterior  Vena  Cava.  This  great  vein  enters  the  thorax 
through  the  foramen  dextrum  of  the  diaphragm.  It  passes  directly 
forwards  to  terminate  in  the  night  auricle,  being  included  between  the 
main  mass  of  the  right  lung  and  its  internal  lobule,  and  placed  at  the 
upper  edge  of  a  double  serous  fold  belonging  to  the  right  pleural  mem- 
brane.    The  right  phrenic  nerve  is  in  company  with  it. 

The  Right  Lymphatic  Duct.     This  is  a  short  lymphatic  vessel  (not 

*  The  vein  is  in  the  natiiral  position  in  Plate  25.  In  Plates  24  and  26  it  is  represented  as  smaller 
than  natural,  and  slightly  lowered  in  position,  in  order  to  expose  the  arteries. 


120 


THE   ANATOMY   OF   THE   HORSE. 


more  than  two  inches)  which  empties  itself  into  the  initial  part  of  the 
anterior  vena  cava,  at  the  angle  of  junction  of  the  jugular  and  axillaiy 


Fig.  7. 
Floor  of  the  Thorax. 
Nos.  1  to  8  indicate  the  corresponding  ribs.    Nos.  9  to  12,  the  cartilages  of  the  corresponding  ribs. 
A.  Cariniform  cartilage  ;  B.  Internal  thoracic  artery  ;  C.  Asternal  artery ;  D.  Triangularis  sterni  ; 
E.  An  internal  intercostal  muscle  ;  F.  Rim  of  diapljragm  ;  G.  Ensiforni  cartilage. 


DISSECTION   OF    THE   BACK   AND    THORAX.  121 

veins  of  the  right  side.  Its  opening  is  provided  with  a  valve.  It 
discharges  into  the  venous  system  the  lymph  collected  in  the  right 
anterior  half  of  the  animal,  viz.,  the  right  fore  limb,  and  the  right 
side  of  the  head,  neck,  chest-wall,  and  diaphragm. 

Directions. — Cut  out  the  trachea  and  lungs  with  the  heart  and  great 
vessels.  Sever  the  lungs  from  the  heart  by  cutting  the  great  vessels  at 
the  root,  and  set  both  lungs  and  heart  aside  in  carbolic  or  other  preser- 
vative solution  to  serve  in  the  examination  of  the  structiu-e  of  these 
organs.  Or,  since  they  are  likely  to  be  much  decomposed,  it  will  be 
better  to  discard  them  if  fresh  organs  can  be  obtained.  In  the  mean- 
time separate  the  sternum  and  costal  cartilages  as  shown  in  Fig.  7, 
and  dissect  the  triangvilaris  sterni  muscle  and  the  internal  thoracic 
vessels.  Portions  of  the  longus  colli  and  psoas  muscles  which  are 
attached  to  the  lower  face  of  the  dorsal  vertebrse  should  be  noticed. 
The  longus  colli  is  described  at  page  156,  and  the  psoas  muscles  at 
page  325. 

The  Triangularis  Sterni  (Fig.  7).  This  muscle  arises  from  the 
lateral  margin  of  the  thoracic  surface  of  the  sternum,  beginning  at  a 
point  opposite  the  2nd  costal  cartilage,  and  extending  backwards  to  the 
ensiform  cartilage,  from  the  edge  of  which  the  last  few  fibres  arise.  It 
is  inserted  into  the  costal  cartilages  from  the  2nd  to  the  8th  inclusive, 
and  into  an  aponeurosis  on  the  internal  intercostal  muscles.  Its  outer 
edge  is  strongly  serrated.  It  covers  the  internal  thoracic  vessels,  and  is 
lined  by  pleura  on  its  upper  face. 

Action. — It  pulls  the  cartilages  to  which  it  is  attached  inwards  and 
backwards,  and  thus  assists  in  expiration. 

The  Internal  Thoracic  (Mammary)  Artery  (Fig.  7).  This  vessel, 
detached  from  the  axillary  artery  at  the  1st  rib,  descends  on  the  inner 
face  of  that  bone,  and  disappears  beneath  the  triangularis  sterni  muscle. 
When  the  muscle  is  removed,  the  artery  is  seen  to  pass  backwards  at 
the  edge  of  the  sternum,  crossing  the  chondro-sternal  joints.  Over  or 
about  the  8th  of  these  joints  it  divides  into  the  asternal  and  anterior 
abdominal  arteries.  The  asternal  branch  emerges  from  under  cover  of  the 
triangularis  sterni,  and  runs  up  the  cartilage  of  the  9th  rib,  on  the 
thoracic  side  of  the  origin  of  the  diaphragm.  About  the  upper  end  of 
the  cartilage  it  passes  through  the  edge  of  the  diaphragm  to  its 
abdominal  side.  The  anterior  abdominal  artery  dips  down  between  the 
cartilage  of  the  9th  rib  and  the  edge  of  the  ensiform  cartilage,  and 
enters  the  abdominal  wall.  The  collateral  branches  of  the  internal 
thoracic  are : — 

1.  Branches  to  the  mediastinum  and  pericardium. 

2.  Pectoral  branches,  perforating  the  intercostal  space  and  anasto- 
mosing with  the  external  thoracic  artery. 

3.  Intercostal  branches,  which  ascend  to  anastomose  with  the  inter- 


122 


THE  ANATOMY  OF  THK  HORSE. 


costal  corteries.  This  series  is  continued  by  branches  of  the  asternal 
artery. 

The  Internal  Thoracic  Vein  runs  in  company  with  the  artery,  and 
internal  to  it.  Beneath  the  triangularis  stemi  it  is  placed  between  the 
artery  and  the  fibrous  cord  that  traverse  the  edge  of  the  sternum. 

Suprasternal  Lymphatic  Glands.  These  include  (1)  a  group  of 
glands  on  the  thoracic  side  of  the  insertion  of  the  diaphragm  across  the 
ensiform  cartilage,  and  (2)  some  small  scattered  glands  along  the  course 
of  the  internal  thoracic  vessels. 


examination  of  the  lung. 
Physical  Characters. — The  exterior  of  the  lung  is  exquisitely  smooth 
in   virtue   of  its    pleural   covering.     Through    this   thin,    transparent 

covering,  the  surface  is  seen, 
especially  when  the  lung  is  dis- 
tended, to  be  divided  by  inter- 
secting lines  into  small  areas,  each 
(if  which  coxTesponds  to  a  lobule 
of  the  lung.  The  lines  are 
formed  by  the  interlobular  con- 
nective-tissue. 

The  colour  of  the  lung  varies 
with  the  age  of  the  animal.  In 
the  young  subject  it  is  pale  pink, 
but  in  old  animals  it  is  of  a 
grayish  or  slaty  hue.  In  the 
foetus  it  is  a  bright  pink. 

The  lung  is  spongy  to  the 
touch,  and  its  cut  surface  has 
the  same  appearance.  It  is  also 
markedly  elastic,  this  quality 
being  best  illustrated  by  the 
rapidity  with  which  the  inflated 
lung  collapses  when  the  distend- 
ing force  is  removed.  It  crepi- 
tates on  pressure  with  the 
fingers,  and  it  floats  on  water. 
The  foetal  lung  is  non-crepitant, 
and  sinks  in  water. 

Structure. — When  the  bronchus  enters  the  lung,  it  divides  again  and 
again  until  there  results  a  remarkable  tree-like  arrangement  of  bronchial 
tubes.  Of  this  tree,  the  bronchus  entering  the  root  of  the  lung  forms 
the  main  stem ;  and  as  the  division  is  traced  onwards,  the  bronchial 
tubes,   representing  the  branches,   become   smaller   and   smaller,  until 


Fig.  8. 

Termination  of  the  Air  Passages  in  the  Lunc 
(modified  from  Tamer). 
A,  A.  Terminal  bronchioles  ;  B.  An  inf  undibuhim, 
showing  the  air-cells  on  its  surface  ;  0.  Pulmonary 
ai-tery  ;  D.  Pulmonary  vein  ;  E.  Pulmonary  capil- 
laries. 


DISSECTION    OF    THE    BACK    AND    THORAX.  123 

there  is  reached  a  tube  of  comparatively  small  calibre  which  belongs 
exclusively  to  one  lobule,  and  is  therefore  termed  a  lobular  or  terminal 
bronchus.  The  left  bronchus  has  a  length  of  three  or  four  inches  before 
dividing,  but  the  right  immediately  gives  off  from  its  outer  side  a  con- 
siderable branch  (Plate  26).  Within  each  lobule  the  terminal  bronchus 
ramifies,  forming  smaller  tubes  or  hronchioles,th.e  last  and  smallest  of  which 
lead  into  recesses  or  dilatations.  Each  such  dilatation  is  termed  an 
alveolar  passage,  and  it  is  bounded  by  delicate  sacculated  walls, 
each  sacculation  being  an  infundibulum.  The  infundibula  are  them- 
selves sacculated,  the  minute  recesses  of  their  walls  being  termed 
air-cells.  The  air-cell  is  thus  the  ultimate  part  of  the  air  passages  within 
the  lung,  and  a  group  of  air-cells  forms  an  infundibulum.  The  wall 
of  an  air-cell  consists  of  a  delicate  membrane  supporting  the  capillary 
plexus  of  the  pulmonary  vessels,  and  lined  towards  the  air  passage  by  a 
single  layer  of  squamous  cells.  The  bronchial  tubes  comprise  in  their 
walls :  (1)  an  outer  fibro-elastic  coat  sustaining  segmented  rings  of 
cartilage;  (2)  within  the  preceding,  a  complete  coat  of  non-striped 
muscular  fibres  circularly  arranged  ;  (3)  an  inner  fibro-elastic  coat ;  (4) 
a  mucous  membrane  with  a  ciliated  epithelium  on  its  free  surface. 
Numerous  mucous  glands  lie  in  the  outer  fibrous  coat,  and  discharge 
their  secretion  into  the  bronchus.  The  bronchi  in  their  ramifications 
are  accompanied  by  divisions  of  the  pulmonary  artery  and  veins,  these 
two  sets  of  vessels  being  connected  by  the  capillary  plexus  on  the  air- 
cells.  Along  the  bronchi  run  also  the  much  smaller  branches  of  the 
bronchial  vessels,  as  well  as  nerves  and  lymphatics. 

Connective-tissue  forms  a  framework  for  the  lung.  It  surrounds 
and  connects  the  bronchi  and  vessels  as  they  run  together  in  the  lung 
substance ;  it  connects  and  isolates  the  adjacent  lobules  ;  and  beneath 
the  pleura  it  forms  a  fibrous  capsule  for  the  lung.  Lymphatic  vessels 
are  abundantly  distributed  in  it,  and  form  three  principal  sets,  viz.,  sub- 
pleural,  perivascular  (around  the  pulmonary  vessels),  and  peribronchial. 

DISSECTION    OP    THE    HEART. 

The  Vessels  of  the  Heart. 

The  Coronary  Arteries  (Plates  23  and  24)  carry  arterial  blood  to 
nourish  the  heart-wall.  They  are  two  in  number,  distinguished  as  right 
and  left.  Each  arises  from  the  common  aorta,  and  has  its  mouth  in 
one  of  the  sinuses  of  Valsalva. 

The  Right  Coronary  Artery  passes  forwards  to  the  right  of  the  pul- 
monary artery  at  its  root ;  and  encircling  the  right  auricular  appendix, 
it  places  itself  in  the  auriculo-ventricular  furrow,  in  which  it  passes  to 
the  right  side  of  the  heart.  On  reaching  the  origin  of  the  right  ventri- 
cular fuiTOw,  it  divides,  one  branch  descending  in  that  furrow,  while  the 
other  continues  the  course  of  the  main  trunk  in  the  auriculo-ventricular 


124  THE  ANATOMY  OF  THE  HORSE. 

groove.  The  terminal  twigs  of  the  vertical  branch  enter  the  heart 
a  little  above  the  apex ;  the  horizontal  branch  reaches  as  far  as  the 
posterior  border  of  the  heart. 

The  Left  Coronary  Artery  passes  outwards  and  to  the  left,  between 
the  pulmonary  artery  and  the  left  auricular  appendix.  Reaching  the 
aviriculo-ventricular  furrow  at  this  point,  it  divides  into  a  vertical  and  a 
horizontal  branch.  The  former  descends  in  the  left  ventricular  furrow  ; 
the  latter  turns  backwards  along  the  auriculo-ventricular  furrow.  The 
terminal  portion  of  the  vertical  branch  turns  round  the  anterior  border 
of  the  heart,  and  ends  in  twigs  that  enter  the  ventricular  wall  on  the 
right  of  its  apex  ;  the  horizontal  branch  terminates  in  the  same  way 
near  the  posterior  border  of  the  heart. 

The  corresponding  branches  of  the  right  and  left  arteries,  thus,  approach 
each  other  at  their  terminations,  but  they  do  not  anastomose ;  nor  is 
there  any  anastomosis  between  the  arteries  through  their  collateral 
branches.  Still  more,  there  is  no  anastomosis  between  the  adjacent 
collateral  branches  of  the  same  coronary  artery.  * 

The  Coronary  Veins. — These  arise  from  the  capillaries  of  the  coronaiy 
arteries.  The  principal  vessel  of  the  right  side  ascends  in  the  right 
ventricular  groove,  and  at  the  auriculo-ventricular  ftirrow  it  joins  the 
main  vein  of  the  left  side.  The  latter  ascends  at  first  in  the  left  ventri- 
cular furrow,  at  the  top  of  which  it  enters  the  auriculo-ventricular 
groove.  Along  this  it  is  reflected  backwards ;  and  turning  round  the 
posterior  border  of  the  heart,  it  joins  the  right  vein.  The  dilated  vessel 
resulting  from  this  union  is  termed  the  coronary  venotis  sinns,  and  it 
opens  into  the  right  auricle,  beneath  the  mouth  of  the  posterior  vena 
cava. 

For  the  most  part,  the  veins  arising  in  the  wall  of  the  right  auricle 
do  not  join  the  large  coronary  veins,  but  open  into  the  cavity  indepen- 
dently, by  minute  mouths — the  foramina  Thebesii. 

The  Nerves  of  the  Heart  have  already  been  described  (pages  110  and 
117).  In  the  heart  of  a  lean  subject  long  filaments  are  visible  without 
dissection,  descending  beneath  the  serous  covering. 

The  Interior  of  the  Heart. — The  cavities  of  the  heart  should  be 
studied  in  the  order  in  which  the  blood  passes  through  them,  and 
therefore  the  right  auricle  fixlls  to  be  examined  first.  The  termina- 
tions of  the  anterior  and  posterior  cavse  and  vena  azygos  (provided 
that  has  an  independent  opening)  should  be  identified  ;  and  then  an 
incision  should  be  made  along  the  wall  of  the  sinus  venosus,  from 
the  opening  of  the  anterior  to  that  of  the  posterior  cava.  Another 
incision  should  be  carried  from  this  one  to  the  point  of  the  auricula ; 

*  Percivall,  Leyh,  Chfinveau,  and  all  the  other  authors  that  I  am  acquainted  with  state  that  the 
coronary  arteries  anastomose  with  one  another.  That  they  do  not,  I  have  reijeatedly  proved  by 
injecting  one  of  them,  by  wlijch  method  none  of  the  injection,  however  fine,  can  be  driven  into  tlie 
other  artery. 


DISSECTION    OP   THE   BACK   AND   THORAX. 


125 


and  the  clots   of  blood   having   been   cleared  out,  the  cavity  will  be 
ready  for  examination. 

The  Cavity  of  the  Right  Auricle.  The  interior  of  this  and  the 
other  cavities  of  the  heart  is  smooth  and  glistening  in  virtue  of  an 
endothelial  membrane  termed  the  endocardium,  which  is  here  continuous 
with  the  endothelial  lining  of  the  great  veins.  It  will  be  observed  that 
the  muscular  wall  of  the  avu-icle  is  thrown  into  parallel  ridges,  which 
from  their  resemblance  to  the 
teeth  of  a  comb  are  termed 
musculi  ])ectinati.  The  venous 
orifices  by  which  the  blood  is 
poured  into  the  cavity  are  all 
found  in  the  sinus  venosus, 
and  are  as  follows : — 1.  The 
anterior  vena  cava  empties 
itself  into  the  anterior  part  of 
the  roof  of  the  sinus.  2.  The 
posterior  vena  cava  discharges 
itself  at  the  lower  and  back 
part  of  the  outer  wall  of  the 
sinus.  3.  The  coronaiy  ven- 
ous sinus  conveys  the  blood 
from  the  wall  of  the  heart  it- 
self, and  its  mouth  will  be 
found  under  that  of  the  pos- 
terior vena  cava.  4.  The  vena 
azygos  sometimes  has  an  in- 
dependent opening  into  the 
auricle,  and  it  then  discharges 
itself  by  the  roof  of  the  sinus, 
behind  the  mouth  of  the 
anterior  cava.     At  other  times 


Fig.  9. 


Diagram  of  the  two  Cavities  of  the  right  side 
OF  THE  Heart  (Ellis). 

a.  Anterior  cava ;  b.  Posterior  cava  ;  c.  Right  auriculo- 
ventrioular   opening ;   d.  Fossa   ovalis  ;  e.  Opening  of 
the  coronary  sinus ;  /'.  Foramina  Thebesii,  the  openings 
.     ,          , ,  ,       .  of  veins ;  rt.   Aperture   of    the  pulmonary  artery ;    h. 

it     opens       into      the     anterior    Auricular- appendix. 


cava.        5.    The   vence    co7'dis 

minivice  are  small  veins  of  the  wall  of  the  right  auricle,  which,  instead 

of   discharging    themselves  by   the   coronary   sinus,  open   directly  on 

the  wall  by  minute   mouths  named  the    foramina  Thebesii.       Of  all 

these  orifices,  that  of  the  coronary  sinus  is  the  only  one  provided  with 

a  valve.     It  is  a  thin  fold  of  the  lining  membrane,  termed  the  valve  of 

Thebesius. 

The  inner  wall  of  the  sinus  venosus  is  formed  by  the  auricular  sep- 
tum, which  is  the  partition  between  the  two  auricles.  On  this  the 
following  objects  are  to  be  noticed : — 1.  Between  the  orifices  of  the 
anterior  and  posterior  cavse  is  a  muscular  prominence — the  tubercle  of 


126  THE  ANATOMY  OF  THE  HORSE. 

Lower.  2.  Above  and  in  front  of  the  opening  of  the  postei'ior  cava  is  a 
depression  of  the  septum  that  looks  like  another  venous  orifice.  This  is 
the  fossa  ovalis,  and  it  marks  the  former  position  of  the  foramen  ovale — 
an  aperture  which,  in  the  foetus,  established  a  communication  between 
the  right  and  left  am-icles.  The  raised  border  which  suiTounds  the 
fossa  is  termed  the  annulus  ovalis.  In  the  foetus  of  many  animals, 
but  not  of  the  horse,  a  valve,  termed  the  Eustachian  valve,  directs  the 
blood  from  the  posterior  cava  through  the  foramen  ovale.  After  birth 
the  foramen  ovale  in  nearly  every  case  becomes  completely  closed,  but 
sometimes  an  oblique  slit  remains,  which,  however,  does  not  necessarily 
permit  any  blood  to  pass  through  the  septum. 

The  blood  which  passes  through  the  right  auricle  is  venous  in  char- 
acter. It  has  been  circulating  among  the  tissues,  and  it  is  poured  into 
the  cavity  at  the  venous  orifices  already  enumerated.  When  the 
auricle  contracts,  the  blood  is  passed  into  the  ventricle  of  the  same  side, 
by  a  large  aperture  of  communication  between  the  two  cavities — the 
right  auriculo-ventrictdar  opening. 

Directions. — The  fore  and  middle  fingers  of  the  left  hand  should  be 
introduced  through  the  auriculo-ventricular  opening,  so  as  to  grasp, 
between  the  fingers  and  thumb,  the  wall  of  the  right  ventricle  close  to 
the  angle  of  junction  between  the  right  ventricular  and  the  auriculo-ventri- 
cular groove.  The  scalpel  should  then  with  the  right  hand  be  passed 
through  the  wall  of  the  ventricle  at  that  point,  and  can-ied  downwards 
in  front  of  the  right  ventricular  furrow;  and  following  that  fuiTOW 
round  the  anterior  border  of  the  heart,  the  incision  should  be  continued 
up  in  front  of  the  left  ventricular  furrow,  as  far  as  the  root  of  the 
pulmonary  arteiy.  This  will  enable  nearly  the  entire  wall  of  the  right 
ventricle  to  be  raised  as  a  triangular  flap,  and  will  give  a  good  view  of 
the  cavity  when  looked  into  from  below. 

The  Cavity  of  the  Right  Ventricle.  This  cavity  is  widest  above, 
and  tapers  to  its  lowest  point ;  and  its  shape  is  such  that  its  transverse 
section  gives  a  crescentic  outline,  the  wall  of  the  ventricle  being  concave 
towards  the  cavity,  while  the  septum  is  convex  in  the  same  direction. 
The  inner  surface  of  its  wall  is  rendered  irregular  by  muscular  bands 
and  prominences — the  columnce  carneca,  of  which  there  are  several 
varieties :  1.  Some  of  them  have  the  form  of  bars  or  ridges  sculptured 
on  the  wall  of  the  heart,  to  which  they  give  a  sponge-like  appearance. 
2.  Others,  the  trabeculce  carnece,  are  veritable  bands  or  strings  between 
which  and  the  wall  of  the  ventricle  the  handle  of  a  scalpel  may  be 
passed.  Of  this  variety  two  or  three  very  tendinous  strings,  sometimes 
more  or  less  reticulate,  stretch  between  the  wall  and  the  middle  of  the 
septum ;  and  since  they  ai'e  believed  to  prevent  over-distension  of  the 
ventricle,  they  have  been  named  moderator  bands.  Other  strings  occur 
in  the  angle  of  junction  of  Avail  and  septum,  and  still  others  stretch 


DISSECTION    OF    THE    BACK    AND    THORAX.  127 

between  different  parts  of  the  wall.  3.  A  third  variety  are  blunt, 
nipple-shaped  prominences  called  musmli  ixvpillares^  of  which  there  are 
commonly  three  in  this  cavity,  one  being  placed  on  the  wall  and  two  on 
the  septum.  Radiating  from  each  of  these  is  a  set  of  fibrous  strings — 
the  chordce  tendinece,  which  are  attached  by  their  other  ends  to  the  seg- 
ments of  the  valve  guarding  the  auriculo-ventricular  opening.  The 
right  auriculo-ventricular  opening  is  situated  at  the  base  of  the  cavity, 
and  is  a  very  large  orifice.  It  is  provided  with  a  valve  composed  of 
three  main  cusps,  or  segments,  and  hence  named  tricuspid.  Each  of  these 
cusps  is  triangular  in  shape,  being  fixed  by  its  base  to  the  wall  of  the 
heart,  and  having  its  edges  free  and  directed  towards  those  of  the 
adjacent  cusps.  When  the  blood  stream  is  rushing  through  the  opening, 
the  segments  of  the  valve  hang  down  into  the  ventricle,  and  have  one 
surface  directed  towards  the  blood  stream,  and  the  other  to  the  wall  of 
the  ventricle.  The  first  of  these  surfaces  is  smooth ;  the  other  is  rough, 
and  to  it  and  the  apex  and  edges  of  the  cusp,  the  chordae  tendineee  are 
attached.  When,  during  contraction  of  the  ventricle,  the  blood  tends 
to  regurgitate  through  the  opening,  the  cusps  are  floated  upwards,  and, 
meeting  each  other,  close  the  orifice.  To  the  efficiency  of  this  action, 
the  chordae  tendinese  passing  from  the  musculi  papillares  are  essential ; 
for,  being  attached  to  the  edges  and  lower  surfaces  of  the  cusps,  they 
prevent  the  latter  from  being  carried  right  up  into  the  auricle.  The  three 
principal  cusps  generally  alternate  around  the  opening  Avith  three  of 
much  smaller  size.  There  are  three  musculi  papillares,  each  with  its 
set  of  chordae  tendineee,  and  three  large  cusps ;  but  it  will  be  observed 
that  one  set  of  the  chordae  tendineae  does  not  pass  entirely  to  one  cusp, 
but  divides  itself  between  two  adjacent  segments. 

Directions. — A  better  view  of  the  tricuspid  valve  may  now  be  obtained 
by  cutting  through  the  auriculo-ventricular  ring  near  the  point  where 
the  first  incision  was  begun  in  opening  the  ventricle,  selecting  the  inter- 
val between  two  cusps.  When  some  of  the  chordae  tendineae  have  been 
cut,  this  will  enable  the  wall  of  the  ventricle  to  be  throw^n  outwards. 

When  the  ventricle  contracts,  the  blood,  prevented  by  the  tricuspid 
valve  from  passing  back  into  the  auricle,  is  forced  upwards  into  the  left- 
anterior  portion  of  the  ventricle,  and  leaves  the  cavity  by  the  pulmonary 
artery.  This  portion  of  the  cavity,  which  leads  up  to  the  artery,  is  termed 
the  conus  arteriostis.  The  orifice  of  the  pulmonary  artery  is  surrounded  by 
a  valve  composed  of  three  crescentic  segments,  and  hence  termed  the 
semilunar  valve.  The  convex  border  of  each  segment  is  fixed  to  the 
wall  of  the  artery  where  it  springs  from  the  ventricle.  The  concave 
border  is  free,  and  shows  at  its  mid  point  a  minute,  fibro-cartilaginous 
thickening — the  nodulus  or  corjms  Arantii.  On  each  side  of  the  corpus 
a  small  cresentic  portion  near  the  free  edge  of  the  segment,  and  dis- 
tinguished from  the  rest  by  its  thinness,  is  termed  the  lunula.      One 


128 


THE  ANATOMY  OP  THE  HORSE. 


surface  of  the  valve  is  convex,  and,  during  contraction  of  the  ventricle, 
it  is  directed  to  the  blood-stream ;  the  other  is  concave,  and  directed  to 
the  wall  of  the  ai-tery,  which,  opposite  each  segment,  forms  a  pouch — 
the  sinvs  of  Valsalva.  When  the  ventricle  has  ceased  to  contract,  the 
elastic  recoil  of  the  artery  forces  the  blood  against  the  concave  side  of 
the  segments,  and  carries  them  inwards  till  they  meet  and  completely 
close  the  opening.  The  blood  is  thus  propelled  along  the  pulmonary 
arteries  to  the  lung,  where,  in  the  capillary  plexus  on  the  walls  of  the 
air-cells,  it  is  purified.  The  purified  fluid  is  then  carried  from  the  lungs 
by  the  pulmonary  veins,  which  pour  it  into  the  left  auricle  of  the  heart. 
Directions. — The  cavity  of  the  left  auricle  is  to  be  exposed  by  an 
incision  from  the  right  to  the  left  pulmonary  veins,  and  by  another 
from  the  first  to  the  point  of  the  appendix. 

The  Cavity  of  the  Left  Auricle  is  smaller  than  the  right,  but,  like 
it,  consists  of  a  simis  venosus  and  an  ear- 
shaped  appendage — the  auricula.  The  pul- 
monary veins  open  on  the  roof  of  the 
sinus  venosus ;  and  most  commonly  they 
have  four  openings — two  from  each  lung, 
but  they  may  have  as  many  as  eight. 
They  are  not  provided  with  valves.  The 
auricula  and  adjacent  part  of  the  sinus 
venosus  show  musculi  pectinati  similar  to 
these  of  the  right  auricle.  In  the  floor  of 
the  cavity  is  the  leji  auric ulo-ve7itricular 
opening,  by  which,  on  contraction  of  the 
auricle,  the  blood  is  passed  into  the  left 
ventricle. 

Directiojis. — The  left  ventricle  should  be 
opened  by  an  incision  similar  to  that  used 
on  the  right  side.  The  point  of  the  scalpel 
shovild  be  passed  through  the  wall  of  the 
ventricle  near  the  upper  end  of  the  left 
ventricular  furrow,  and  the  incision  should 
be  carried  doAvn  the  left  side  of  the 
round   the    apex,    and   up    the 


Fig.  10 


Diagram  ok  the  two  Cavitie 

THE   LEFT   SIDE   OK   THE    HEART  (Ellis).  ,     .     ■, 

,  ^  .       ,  •       •   T,-  1  /  ventricle, 

k.  Left  pulmonary  veins ;  i.   Right  '                         . 

pulmonary  veins ;  o.  Remains  of  fora-  right    sidc     tO    WlthlU    a    short    dlStaUCe     Of 

men  ovale;    «.  Left  auriculo-ventiic-  °              .       ,              ,    •       i                                ,i 

ular  opening  ;  m.  Auricidar  appendix  ;  the     auriCUlo-VeiltriCUlar     grOOVC,     the     CUt 

n.  Aperture  of  the  aorta.  ^^.^^  ^^^^  ^^g.^^.  ^^^    geptum,  tO    which  the 

ventricular  furrows  will  serve  as  a  guide. 

The  Cavity  of  the  Left  Ventricle  is  longer  than  the  right,  and  is 
almost  conical  in  shape,  the  base  being  at  the  auriculo-veutricular 
opening.  On  transverse  section,  it  gives  an  oval  or  nearly  circular 
outline,  the  septum,  as  well  as  the  wall  of  the  ventricle,  being  concave 


DISSECTION    OF    THE    BACK    AND    THORAX. 


12J 


towards  the  cavity.  It  will  be  observed  that  its  wall  is  about  thrice 
the  thickness  of  that  of  the  right  cavity,  a  circumstance  which  makes 
it  easy  to  distinguish  the  right  and  left  ventricles  in  the  undissected 
heart ;  for,  whereas  the  former  appears  flabby,  the  latter  is  firm  and 
solid-looking.  The  left  ventricle  possesses  columnoe  carneoi  like  those  on 
the  right  side.  The  musculi  papillares  are  two  in  number,  and  are  of 
very  large  size.  They  are  placed  on  the  wall,  and  are  provided  with 
chordce  tendinece  stronger  than  those  of  the  right  cavity.  The  base  of 
the  cavity  shows  the  left  auriado-ventricular  opening,  which  is  somewhat 
smaller  than  the  right.  It  is  guarded  by  a  valve  with  two  large  cusps, 
and  hence  called  the  bicuspid  valve.  It  is  also  very  commonly 
designated  the  mitral  valve,  from  a  fancied  resemblance  to  a  bishop's 
mitre.  The  cusps  are  stronger  than  those  of  the  tricuspid  valve,  with 
which  they  agi-ee  in    shape  and  disposition.     Two    smaller   segments 


Fig.  11. 

Root  of  the  common  Aorta  laid  open. 

1,  1,  1.  Semilunar  segments  of  the  aortic  valve  ;  2.  Corpus  Arantii ;  3,  3.  Orifices  of  right  and  left 

coronary  arteries  from  two  of  the  sinuses  of  Valsalva  ;  4.  Ventricular  wall ;  5.  Arterial  wall. 

alternate  with  the  main  ones.  In  mode  of  action  the  mitral  exactly 
resembles  the  tricuspid  valve.  When  the  ventricle  contracts,  the  blood, 
prevented  from  regurgitating  into  the  auricle,  is  forced  out  of  the 
cavity  along  the  great  systemic  arteiy — the  common  aorta,  which  springs 
from  the  right-anterior  part  of  the  base  of  the  ventricle.  The  aortic 
orifice  is  guarded  by  a  three-segmented  semihmar  valve.  These  segments 
are  stronger  than  those  at  the  mouth  of  the  pulmonary  artery,  which 
they  otherwise  exactly  resemble.  Opposite  to  each  a  large  sinus  of 
Valsalva  is  developed  on  the  wall  of  the  artery,  and  from  two  of  these 
spring  the  right  and  left  coronary  arteries  of  the  heart. 

STRUCTURE   OP   THE    HEART. 

In  structure  the  heart  consists  of  a  muscular  wall,  an  external  serous 
investment — the  epicardiwn,  and  an  internal  serous  lining — the  endocar- 
dium.    The  valves  are  folds  of  the  endocardium,  strengthened  with  fibrous 


130  THE  ANATOMY  OF  THE  HORSE. 

conuective-tissue,  to  which  are  added  some  elastic  fibres.  The  muscuhxr 
tissue  is  of  the  striped  variety  (although  not  under  the  control  of  the  will), 
and  its  fibres  are  grouped  in  bundles  separated  by  fibrous  connective- 
tissue.  Connective-tissue  occurs  also  in  large  amount  in  the  neighbour- 
hood of  the  auriculo-ventricular  and  arterial  openings,  where  it  is 
aggregated  in  the  form  of  rings,  or  zones.  These  rings  give  to  the 
orifices  that  firmness  which  is  necessary  for  the  efficient  working  of  the 
valves,  and  at  the  same  time  give  origin  to  some  of  the  muscular  fibres. 
The  tissue  of  which  they  are  composed  is  mainly  fibro-cartilaginous. 
In  the  heart  of  the  ox,  and  rarely  also  in  the  horse,  a  bone — the  os 
cordis — is  developed  in  the  angle  between  the  aortic  ring  and  the  two 
auriculo-ventricular  rings. 

Directions. — The  arrangement  of  the  muscular  tissue  in  the  wall  of 
the  heart  is  exceedingly  complex,  and  cannot  be  studied  except  in  a 
heart  specially  prepared.  A  heart  from  any  of  the  domestic  animals, 
but  preferably  from  the  horse,  should  be  procured,  and  boiled  for  about 
an  horn-.  This  will  favour  the  dissection  of  the  fibres,  by  making  them 
firm  and  softening  the  connective-tissue  between  them.  The  epicardium, 
fat,  and  vessels  having  been  cleaned  off"  the  surface  of  the  heart,  the 
auricles  should  be  first  examined. 

The  auricles  have  the  muscular  fibres  of  their  walls  distinct  from 
those  of  the  ventricles.  Moreover,  the  fibres  are  arranged  in  two  layers — 
a  deep  set  proper  to  each  auricle,  and  a  superficial  set  common  to  both, 
some  of  the  fibres  of  the  latter  stratum  being  carried  into  the  auricular 
septum.  In  the  deep  stratum  some  of  the  fibres  run  obliquely  in  the 
wall,  while  others  are  arranged  as  rings  around  the  auricula  and  the 
diff'erent  venous  orifices,  the  latter  playing  an  important  part  in  pre- 
venting regurgitation  into  the  veins  when  the  auricle  contracts. 

Directions. — Separation  of  the  auricles  from  the  ventricles  should  next 
be  effected  by  cutting  the  auriculo-A-entricular  fibrous  rings,  which  form 
the  bond  of  connection  between  the  auricular  and  ventricular  fibres. 
By  combined  cutting  and  teasing  the  following  facts  may  be  observed. 

Over  the  whole  exterior  of  the  ventricles  the  fibres  have  an  oblique 
direction.  Thus,  on  the  left  side  the  fibres  pass  obliquely  downwards  and 
backwards,  and  on  the  right  side  downwards  and  forwards.  At  the  left 
ventricular  furrow  many  of  the  fibres  dip  into  the  septum;  but  on  the 
right  side  the  fibres  of  the  left  ventricle  pass  across  the  furrow,  and  are 
directly  continued  on  the  right  ventricle.  At  the  apex  of  the  heart 
the  fibres  turn  inwards  in  a  whorl-like  manner  and  disappear  from  view. 
If  a  thin  stratum  of  these  superficial  fibres  be  now  removed,  they  will 
be  found  to  cover  others  having  a  less  oblique  course  ;  and  further 
dissection  will  show  that  the  fibres  become  less  and  less  oblique  until 
the  centre  of  the  wall  is  reached,  where  the  fibres  are  approximately 
horizontal.     On  peeling  off"  these  horizontal  fibres,  a  deeper  set  will  be 


DISSECTION    OF    THE    BACK    AND    THORAX,  131 

exposed  which  are  slightly  inclined,  but  in  a  direction  opposite  to  the 
superficial  fibres;  and  the  obliquity  of  these  increases  until  the  inner 
surface  of  the  wall  is  reached,  where  the  fibres  have  a  degree  of  obliquity 
equal  to  the  most  external  fibres.  So  much  the  student  will  probably 
be  able  to  make  out  without  much  difficulty,  but  according  to  Pettigrew 


View  of  a  Pabtial  Dissection  of  the   Fibres  of  the   left  Wall  of  the   Ventricles  Tin  a 
Sheep's    Heart,    designed  to   show   the    different   Degrees    of  Obliquity   of_.the 
Fibres  (Allen  Thomson). 
At  the  base  and  apex  the  superficial  layer  of  fibres  is  displayed  :  in  the  intervening  space,  more 
and  more  of  the  fibres  have  been  removed  from  above  downwards,  reaching  to  a  greater  depth  on 
the  left  than  on  the  right  side.     a',  a'.  The  superficial  layer  of  the  right  ventricle  ;  6'.  6'.  The  same 
of  the  left  ventricle  ;  at  2  this  superficial  layer  has  been  removed  so  as  to  expose  the  fibres  under- 
neath, which  are  seen  to  have  the  same  direction  as  the  supei-ficial  ones  over  the  left  ventricle,  but 
diflerent  over  the  right ;  at  3  some  of  these  have  been  removed,  but  the  direction  is  only  slightly 
different ;  4.  Transverse  or  annular  fibres  occupying  the  middle  of  the  thickness  of  the  ventricular 
walls  ;  5,  6,  7.  Internal  fibres  passing  downwards  towards  the  apex  to  emerge  at  the  whorl ;  between 

c.  c.  the  left  ventricular  groove,  over  which  the  fibres  of  the  superficial  layer  are  seen  crossing  ;  in 
the  remaining  part  of  the  groove,  some  of  the  deeper  fibres  turn  backwards  towards  the  septum  ; 

d.  The  pulmonary  artery  ;   e.  The  aorta. 

the  fibres  are  arranged  in  seven  determinate  layers — three  external 
which  are  oblique,  three  internal,  also  oblique  but  in  the  opposite 
direction,  and  a  central  which  is  horizontal.  Further,  he  describes  the 
fibres  of  the  most  external  layer  as  turning  in  at  the  auriculo-ventricular 
orifices  and  at  the  apex  of  the  heart  to  become  continuous  with  the 
layer  beneath  the  endocardium.  In  like  manner,  he  supposes  that  the 
second  layer  is  continuous  with  the  sixth,  and  the  third  with  the  fifth, 
while  the  fourth  or  central  layer  has  a  zone-like  arrangement.  In  truth, 
however,  the  fibres  of  the  same  stratum  anastomose  not  only  with  one 
another,  but  also  with  the  fibres  of  adjacent  strata,  as  is  shown  by  the 


132 


THE  ANATOMY  OF  THE  HORSE. 


rough  sui-ftice  which  is  left  when  one  set  of  fibres  is  removed  from  the 
underlying  set. 

Directions. — The  joints  and  ligaments  of  the  dorso-lumbar  part 
of  the  spinal  column  and  of  the  ribs  must  now  be  dissected. 
The  ligaments  of  the  lumbar  region  will  be  exposed  by  carefully 
removing  from  the  surface  of  the  bones  the  remains  of  muscles 
and  other  textures  already  examined.  The  whole  of  the  dorsal 
region  need  not  be  dissected  in  order  to  expose  the  ligaments,  but  it 
will  sufl&ce  to  take  a  segment  containing  four  or  five  vertebrae  with  their 
costal  articulations  intact.  The  articulations  of  the  costal  cartilages  to 
the  sternum  are  to  be  examined  on  the  part  of  the  thorax  removed  for 
the  display  of  the  triangularis  stemi  muscle. 

ARTICULATIONS   OF   THE   RIBS. 

Each  rib  is  articulated  to  the  spinal  column  at  two  points,  viz., 
by  its  head,  and  by  its  tubercle.  The  head  is  received  into  a  cup- 
like cavity  foi-med  by  two  adjacent  vertebral  bodies  and  the  disc 
that  unites  them.  This  is  the  costo-central  joint.  The  tubercle  articu- 
lates with  the  flat  facet  on  the  transverse  process  belonging  to  the 
posterior  of  the  vertebrae  to  which  the  head  is  articulated.  This  is  the 
costo-transverse  joint. 

CosTO-CENTRAL  JoiNT.  This  possesscs  tw^o  ligaments — the  costo- 
vertebral and  interarticular,  and  two  synovial  sacs. 


Fig.  13. 
two  costo- vertebral,  and  two  intervertebral  joints,  viewed  from  below. 
1.  Attachment  of  costo-vertebral  (stellate)  ligament  to  intervertebral  disc  ;   2.  and  3.  Attachments 
of  the  same  ligament  to  the  anterior  and  posterior  vertebral  bodies ;  4.  Posterior  coKto-transverse 
ligament ;  5.  Intervertebral  disc,  covered  by  6.  the  inferior  common  ligament. 

The  Costo-vertebral  or  Stellate  Ligament  is  placed  beneath  the  joint.  Its 
fibres  radiate  from  the  rib  just  below  its  articular  head,  and  become 
attached  to  the  body  of  the  vertebra  in  front,  to  the  body  of  the 
vertebra  behind,  and  to  the  intermediate  disc. 


DISSECTION    OF    THE    BACK    AND    THORAX.  133 

The  Inter  articular  Ligament  is  fixed  to  the  groove  dividing  the  articular 
head  of  the  rib  into  two  facets.  It  passes  inwards  across  the  floor  of 
the  spinal  canal,  being  united  to  the  upper  edge  of  the  intervertebral 
disc ;  and  on  the  middle  line  it  becomes  continuous  with  the  corre- 
sponding ligament  of  the  opposite  rib.  It  is  not  present  in  the  1st  rib. 
It  should  be  displayed  by  disarticulating  the  costo-transverse  joints  of 
the  first  vertebra  in  the  segment,  and  then  removing  the  arch.  On 
removing  the  superior  common  ligament,  it  will  be  found  in  the  interval 
between  the  two  vertebral  bodies. 

Synovial  Sacs. — There  is  one  sac  on  each  side  of  the  interarticular 
ligament.     There  is  only  one  sac  for  the  first  costo-central  joint. 

CosTO-TRANsvERSE  JoiNT.  This  is  maintained  by  two  ligaments — an 
anterior  and  a  posterior  costo-transverse  ligament,  and  it  possesses  a 
synovial  sac. 


Fig.  14. 

Two   COSTO-VERTEBRAL,    AND  TWO    INTERVERTEBRAL  JOINTS,    VIEWED  FROM   ABOVE.      ThE    LaMIN^ 

OF  THE  Vertebra  have  been  removed. 
1.   Posterior  costo-transverse  ligament ;   2.   Anterior    (interosseous)    costo-transverse    ligament 
3,  3.  Superior  common  ligament. 

The  Postei'ior  Costo-transverse  Ligament  is  composed  of  a  band  of 
fibres  stretching  across  the  joint  behind,  and  fixed  by  its  extremities  to 
the  rib  and  transverse  process. 

The  Anterior  {Interosseous)  Costo-transverse  Ligament  stretches  between 
the  antero-inferior  aspect  of  the  transverse  process  and  the  neck  of 
the  rib.  It  is  partly  concealed  by  the  transverse  process,  and  is  best 
seen  when  viewed  from  above  and  in  front. 

Synovial  Sac. — This  will  be  exposed  by  removing  the  posterior  costo- 
transverse ligament.  In  the  last  two  or  three  ribs  there  is  no  separate 
synovial  sac  for  the  costo-transverse  joint ;  but  the  posterior  costo- 
central  sac  is  extended  over  it,  the  two  articular  surfaces  being  in 
these  ribs  continuous. 


134  THE  ANATOMY  OF  THE  HORSE. 

Chondro-Costal  Joint,  The  inferior  extremity  of  the  rib  is  slightly 
excavated,  and  receives  the  extremity  of  the  costal  cartilage.  The 
periosteum  passes  from  the  rib  to  the  cartilage,  and  serves  to  consolidate 
the  union. 

Chondro-Sternal  Joint.  This  is  the  joint  by  which  the  costal 
cartilage  of  each  of  the  first  eight  ribs  is  articulated  to  the  sternum. 
Peripheral  fibres  envelop  the  joint  and  form  a  cajjsular  ligament. 
Above  and  below  the  joint  the  capsule  is  somewhat  thickened,  forming 
the  superior  and  inferior  costo-sternal  ligaments. 

Synovial  Sac. — The  joint  possesses  a  synovial  membrane.  The 
cartilages  of  the  first  pair  of  ribs  meet  in  a  common  joint  on  the  middle 
line,  and  there  is  a  single  synovial  sac  common  to  these  chondro-stemal 
articulations,  and  to  the  facet  between  the  two  cartilages. 

Union  of  the  Costal  Cartilages  to  one  another.  The  cartilage 
of  the  first  asternal  rib  (9th)  is  firmly  bound  to  the  preceding  carti- 
lage by  short  fibrous  bands.  It  is  further  bound  to  the  lower  face 
of  the  xiphoid  cartilage  by  a  small  band — the  chondro-xiphoid  ligament. 
From  the  tip  of  each  succeeding  cartilage,  a  yellow  elastic  band  is  carried 
to  the  posterior  edge  of  the  cartilage  in  front  of  it. 

Movements  op  the  Eibs.  Each  rib  with  its  cartilage  moves  around  an 
imaginary  axis  joining  the  head  of  the  rib  and  the  sternal  end  of 
the  cai-tilage.  In  inspiration  the  rib  moves  forwards  and  outwards 
round  this  axis,  so  as  to  bring  the  middle  portion  of  the  rib  towards  the 
position  occupied  by  the  preceding  rib  at  the  end  of  expiration.  This 
movement  lengthens  the  line  joining  the  mid  point  of  each  rib  to  the 
con-esponding  point  on  the  opposite  rib,  and  thus  increases  the  capacity 
of  the  chest  by  increasing  its  transverse  diameter.  During  expiration 
the  rib  falls  into  its  original  position  by  moving  in  the  opposite  direction. 
In  these  movements  the  head  of  the  rib  and  the  extremity  of  the  costal 
cartilage  rotate  slightly  in  their  cavities,  but  without  change  of  place. 
The  tubercle  of  the  rib  glides  on  the  facet  of  the  transverse  process, 
moving  in  a  circle  whose  centre  is  the  costo-central  joint. 

The  Sternum.  There  are  no  joints  in  the  sternum  of  the  horse,  in 
which  the  osseous  segments  are  simply  united  by  persisting  portions  of 
the  original  cartilaginous  mass.  In  this  connection,  however,  there  may 
be  noticed  the  two  suprasternal  fibrous  cords  which  pass,  one  at  each 
side  of  the  thoracic  surface  of  the  sternum,  internal  to  the  mammary 


inter-vertebral  joints  and  ligaments. 
In  the  dorsal  and  lumbar  regions  adjacent  vertebrae  are  connected 
(1)  by  an  amphiarthrodial  joint  between  their  bodies,  and  (2)  by 
synovial  joints  between  their  articular  processes.  These  same  joints  are 
formed  between  the  last  lumbar  vertebra  and  the  sacrum;  and,  in 
addition,  there  is  a  synovial  joint  between  the  last  lumbar  transverse 


DISSECTION    OF    THE    BACK    AND    THORAX.  135 

process  and  the  base  of  the  sacrum.  Inter- transverse  joints  are  also 
developed  between  the  4th  and  5th,  and  5th  and  6th  lumbar  transverse 
processes.  The  ligaments  may  be  classified  into  : — 1.  Those  connecting 
the  processes  and  neural  arches  of  adjacent  bones.  2.  Those  connecting 
adjacent  vertebral  bodies. 

Ligaments  of  the  Processes  and  Neural  Arches  : — 

The  Supraspinous  Ligament  is  a  strong  longitudinal  band,  or  cord, 
extending  along  the  tips  of  the  spinous  processes.  It  is  continued  back- 
wards on  the  sacral  spines ;  and  in  the  anterior  part  of  the  dorsal  region 
its  texture  changes  from  white  fibrous  to  yellow  elastic  tissue,  and  is  con- 
tinued forwards  as  the  funicixlar  portion  of  the  ligamentum  nuchfe.  It 
not  only  tends  to  maintain  the  union  of  the  vertebrae,  but  also  affords  a 
point  of  origin  to  muscles  of  the  back  and  loins. 

The  Interspinous  Ligaments  occupy  the  interspaces  of  the  spinous 
processes.  In  each  space  the  ligament  consists  of  a  right  and  a  left  layer 
whose  fibres  have  a  downward  and  backward  direction.  This  oblique 
direction  of  the  fibres  favours  the  separation  of  the  spines  during 
flexion. 

Capsular  Ligaments  of  the  Articular  Processes. — These  complete  the 
diarthrodial  joint  formed  between  the  articular  processes  of  adjacent 
vertebrae,  and  support  the  synovial  sac  of  the  joint.  One  of  the 
ligaments  should  be  slit  open  to  display  the  synovial  membrane  lining 
its  inner  surface. 

The  Ligamenta  suhflava,  or  Ligaments  of  the  Arches. — These  pass  between 
the  edges  of  adjacent  neural  arches.  They  are  best  seen  by  sawing 
horizontally  through  the  pedicles  of  two  vertebrae,  close  to  the  body, 
and  then  pulling  the  arches  apart  while  they  are  viewed  from  below. 

Capsular  Ligaments  of  the  Transverse  Pi-ocesses. — These  surround  the 
joints  developed  between  the  transverse  processes  of  the  4th  and  5th, 
and  5th  and  6th  lumbar  transverse  processes,  and  between  the  last 
of  these  processes  and  the  base  of  the  sacrum.  On  removing  them,  the 
joints  will  be  found  to  possess  a  synovial  membrane.  In  old  subjects, 
however,  these  joints  are  generally  obliterated  by  anchylosis. 

Ligaments  of  the  Bodies  : — 

The  Inferior  Common  Ligament  is  a  thin  stratum  of  fibres  covering  the 
lower  face  of  the  vertebral  bodies  and  the  intervertebral  discs.  It  is  con- 
tinued backwards  beneath  the  sacrum,  but  it  is  not  traceable  as  a  distinct 
ligament  farther  forwards  than  the  6th  dorsal  vertebra. 

The  Superior  Common  Ligament  lies  on  the  floor  of  the  spinal  canal, 
and  must  be  exposed  by  the  removal  of  the  nexiral  arches.  It  is  a 
riband-like  structure  adherent  to  the  vertebral  bodies,  and  to  the  inter- 
articular  ligament  of  the  ribs  or  to  the  upper  edge  of  the  intervertebral 
disc.  The  edges  of  the  ligament  are  scalloped,  the  ligament  being 
widest  where  it  passes  over  the  intervertebral  discs,  and  narrowest  at 


136  THE  ANATOMY  OF  THE  HORSE. 

the  middle  of  the  vertebral  bodies.  It  is  continued  into  the  sacral  and 
coccygeal  regions. 

The  Intei-vertebral  Substance. — Between  every  two  adjacent  vertebral 
bodies  there  is  interposed  a  disc  of  fibro-cartilage.  This  will  be  best 
seen  on  making  a  vertical  mesial  section  of  two  centra.  The  disc  is 
thinner  in  the  back  than  in  the  loins  or  neck.  The  disc  between  the 
last  lumbar  body  and  the  sacrum  is  especially  thick.  In  the  dorsal 
region  they  concur  in  forming  the  cavity  for  the  head  of  a  rib.  The 
disc  is  not  of  unifoi-m  texture  throughout.  The  peripheral  part  of  each 
is  composed  of  alternating  layers  of  fibrous  tissue  and  fibro-cartilage. 
In  each  layer  the  fibres  pass  in  an  oblique  direction  between  the  two 
bones,  and  in  successive  layers  the  fibres  are  alternately  oblique  in 
opposite  directions.  The  central  portion  of  the  disc  is  pulpy,  soft,  and 
elastic ;  and  is  interesting  as  being  a  persistent  portion  of  the  foetal 
chorda  dorsalis.  The  peripheral  part  of  the  disc  constitutes  an  extremely 
resistant  bond  of  union  between  the  two  vertebra),  while  the  central 
pulpy  portion  permits  rotation  of  the  one  bone  on  the  other. 

Movements  of  the  dorso-lumbar  part  of  the  spinal  column.  These  are 
flexion,  extension,  lateral  inclination,  and  rotation.  Flexion  and  extension 
are  opposite  movements  taking  place  in  a  vertical  plane.  In  flexicm  the 
downward  concavity  of  the  column  is  increased,  in  extension  it  is 
diminished.  These  movements  are  much  more  restricted  here  than  in 
the  neck,  owing  to  the  thinness  of  the  intervertebral  discs  and  the  large 
size  of  the  spinous  processes.  They  have  a  greater  range  in  the  loins 
than  in  the  back.  Lateral  bending  is  also  much  less  free  than  in  the 
neck,  being  impeded  by  the  thinness  of  the  intervertebral  substance, 
and  by  the  ribs  and  lumbar  transverse  processes.  Its  greatest  range  of 
movement  is  in  the  anterior  part  of  the  lumbar  region.  Eolation  is  the 
twisting  or  turning  of  a  vertebra  round  a  longitudinal  axis  passing 
through  its  body.  It  is  not  permitted  in  the  lumbar  region,  owing  to 
the  form  of  the  articular  processes ;  and  even  in  the  back,  it  is  scarcely 
appreciable. 

THE    SPINAL    CORD. 

Directions. — To  expose  the  spinal  cord  of  the  horse  in  the  whole  of  its 
extent  is  a  tedious  and  difficult  operation.  Moreover,  where  the  dissec- 
tion of  the  parts  surroimding  the  vertebral  column  is  apportioned 
between  the  dissectors  of  the  neck,  thorax,  abdomen,  and  pelvis,  it  is 
quite  impossible,  without  unduly  interfering  with  what  is  otherwise  the 
most  convenient  course  of  dissection,  to  expose  at  once  the  entire  cord. 
This,  however,  is  not  a  matter  of  much  importance,  since,  in  all  the 
main  features  of  its  structure,  the  spinal  cord  of  the  horse  is  identical 
with  that  of  any  other  mammal.  The  student  is  therefore  advised  to 
study  the  cord  of  a  dog  or  a  cat,  whicli  may  be  exposed  without  diffi- 


DISSECTION    OF    THE    BACK    AND    THORAX.  137 

culty.  One  of  these  animals  having  been  secured  (and  preferably  a  dog), 
it  should  be  fastened  to  a  table  in  the  prone  position,  and  a  mesial  inci- 
sion through  the  skin  and  muscles,  down  to  the  vertebrte,  should  be  made 
from  the  occiput  to  the  root  of  the  tail.  With  the  knife  the  muscles 
are  to  be  reflected  so  as  to  expose  the  vertebi-se  as  far  as  the  junction  of 
the  arch  with  the  body.  The  spinal  canal  is  then  to  be  opened  by 
removing  the  arches  with  a  chisel  and  mallet,  or  with  bone-forceps.  In 
the  dorsal  region  each  arch  must  be  disarticulated  from  its  connection 
with  the  ribs.  The  spinal  cord  enclosed  within  its  membranes  will  now 
be  exposed,  and  between  the  outer  membrane  and  the  bones  are  some 
veins  and  a  quantity  of  fat. 

Membranes,  or  Meninges,  of  the  Spinal  Cord. 

The  Dura  Matbr.  This  is  the  most  external  of  the  membranes.  It 
is  the  protective  envelope  of  the  cord,  and  has  the  form  of  tubular  mem- 
brane of  fibrovis  connective-tissue,  extending  from  the  foramen  magnum, 
where  it  is  continuous  with  the  corresponding  envelope  of  the  brain,  to 
the  posterior  end  of  the  spinal  canal  in  the  coccygeal  region.  It  does 
not  form  a  tight-fitting  covering  to  the  cord,  but  invests  it  somewhat 
loosely.  Its  outer  surface,  it  is  to  be  observed,  is  smooth,  and  does  not 
line  the  vertebrae,  which  have  the  ordinary  periosteal  covering.  It  is 
connected  by  some  slender  fibrous  processes  to  the  superior  common 
ligament.  The  capacity  of  the  tube  varies  with  the  thickness  of  the 
cord,  being  greater  at  the  atlas,  lower  part  of  the  neck,  and  lumbar 
region  than  at  the  intermediate  points.  The  spinal  cord  does  not  extend 
beyond  the  middle  of  the  sacrum,  but  the  dura  mater  is  prolonged 
a  few  inches  beyond  that  as  an  impervious,  tapering  process.  On  each 
side  the  dura  mater  is  perforated  by  the  roots  of  the  spinal  nerves,  and 
along  these  it  sends  offsets  as  far  as  the  intervertebral  foramina. 

Directions. — A  small  piece  of  the  dura  mater  should  be  pinched  up 
with  forceps  and  snipped  through.  Beginning  at  the  slit  thus  formed, 
it  should  be  laid  open  backwai-ds  and  forwards  along  the  middle  line. 
As  this  is  being  done,  the  membrane  should  be  pinched  up,  so  as  to 
prevent  injury  to  the  cord. 

The  Arachnoid  is  the  second  of  the  membranes  of  the  cord.  It  is 
much  more  delicate  than  the  dura  mater,  and  in  disposition  and  struc- 
ture it  is  comparable  to  a  serous  membrane.  Like  such  membranes,  it 
encloses  a  cavity,  or  sac,  and  consists  of  a  2)(iri€tal  and  a  visceral  portion. 
The  sac  is  known  as  the  arachnoid  cavity,  or  suh-dural  space,  receiving 
the  latter  designation  from  its  relation  to  the  dura  mater.  The  parietal 
division  of  the  membrane  is  represented  by  a  layer  of  endothelium  lining 
the  inner  surface  of  the  dura  mater,  to  Avhich  it  gives  a  smooth  and  glisten- 
ing aspect,  but  from  which  it  is  not  separable  by  dissection.  The  vis- 
ceral division  invests  the  cord  and  pia  mater  as  a  thin  transparent  mem- 
brane, but  it  does  so  loosely,  leaving  a  space  between  it  and  the  outer 


138 


THE  ANATOMY  OF  THE  HORSE. 


surface  of  the  pia  mater.  This,  which  is  the  sub-arachnoid  sjmce,  con- 
tains a  variable  amoxiut  of  an  alkaline  fluid — the  sub-arachnoid  fluid, 
which  acts  as  a  water-bed  to  the  cord.  As  the  roots  of  the  spinal  nerves 
extend  outwards,  they  take  with  them 
a  covering  from  the  visceral  arachnoid ; 
and  Avhere  they  pierce  the  dura  mater, 
this  covering  becomes  continuous  with 
the  parietal  layer. 

The  Pia  Mater  is  the  vascular  mem- 
brane of  the  cord.  It  consists  of  areolar 
connective-tissue  in  which  the  vessels 
subdivide  before  entering  the  cord.  It 
invests  the  cord  closely,  and  is  intimately 
connected  to  it;  sending  a  considerable 
process  into  the  inferior  median  fissure, 
and  numerous  other  slender  filaments 
which  penetrate  the  substance  of  the 
cord.  On  each  side  it  is  connected  to 
the  inner  surface  of  the  dura  mater  by 
a  series  of  pointed  processes  constituting 
the  ligamentum  denticulatuni.  Each  of 
these  processes  of  pia  mater  passes  out- 
FiG.  15.  wards  from   the   side  of  the  cord,    and, 

View  ok  the  Membranes  ok  the  Spinal  caiTying  the    arachnoid    with    it,   bcCOmCS 

_  "    ^      '      ,1    a   *  J     attached  to  the  dura  mater,  midway  be- 

a.  Dura  mater  cut  open  and  reflected ;  '  "^ 

6.  Small  part  of  the  translucent  arach-    twcen    the    poiuts     of    perforation    of    the 
noid,  left ;  li.  Pia  mater  closely  investing  '■ 

the  spinal  cord ;  c.  Ligamentum  denticu-  supsrior    and    the    inferior    iicrve-roots. 

latum  on  the  side  of  the  cord,  sliown  by-p,,.,^,  •     ,    •        ^  ^ 

cutting  tiirough  the  inferior  roots  of  the  xJehmd  the  point  in  the  sacral  rcgioii  at 
rt™he''dnrr  nLL'!  ^.'^"Sie"' spiiTal  "which  the  spiual  cord  stops,  the  pia 
cS=SupSrootI°"*'°'*^'""'^'''  mater  is  prolonged  as  an  attenuated 
thread — the  Jiium  tenninale — which  is 
enclosed  by,  and  blends  with,  the  tapering  end  of  the  dura  mater. 

Directions. — Before  the  removal  of  the  cord  the  student  should  observe 
its  varying  thickness  at  different  points,  and  the  disposition  of  the  spinal 
nerves  within  the  spinal  canal. 

The  spinal  cord  begins  at  the  foramen  magnum  by  continuity  with 
the  medulla  oblongata,  and  it  is  here  of  considerable  thickness.  Trac- 
ing it  backwards,  it  is  seen  to  become  thicker  behind  the  middle  of 
the  cervical  region,  forming  the  cervical  enlargement,  which  extends  as 
far  as  the  2nd  dorsal  vertebra.  It  is  from  this  enlargement  that  the 
nerves  which  supply  the  fore  limb  are  given  off;  Beyond  the  2nd 
dorsal  vertebra  the  cord  contracts  slightly,  so  as  to  become  about  the 
middle  of  the  back  smaller  even  than  in  its  initial  portion.  Preserving 
this  diminished  thickness  throughout  the  dorsal  region,  it  again  expands 


DISSECTION    OF    THE    BACK    AND    THORAX.  139 

in  the  lumbar  region,  forming  a  second  swelling — the  lumbar  enlarge- 
ment, from  which  the  nerves  for  the  supply  of  the  hind  limb  are  de- 
tached. Beyond  the  lumbar  enlargement  the  cord  rapidly  becomes 
reduced  in  volume,  and  tapers  to  a  point  about  the  2nd  sacral  seg- 
ment. This  tapering  extremity  of  the  cord — the  comes  medullaris — is 
prolonged  backwards  by  the  filum  terminale,  into  which  its  nervous 
structure  is  continued  for  a  little  distance. 

The  Spinal  Nerves  of  the  horse  number  forty-two  or  forty-three  pairs, 
and  their  number  in  the  different  regions  of  the  vertebral  column  is 
expressed  in  the  following  formula  : — 

CsI^isLgSsCs  or  c- 

The  1st  cervical  nerve  leaves  the  canal  by  the  antero-internal  foramen 
of  the  atlas,  the  2nd  by  the  foramen  in  the  front  of  the  arch  of 
the  axis,  and  the  others  in  succession  pass  out  by  the  intervertebral 
foramina. 

In  the  other  regions  the  nerves  are  numbered  according  to  the 
vertebrse  behind  which  they  emerge ;  thus,  the  1st  dorsal  nerve  emerges 
by  the  intervertebral  foramen  behind  the  1st  dorsal  vertebra,  and  so  on 
with  the  others. 

In  the  cervical  region  the  nerves  pass  nearly  directly  outwards  from 
the  cord  to  their  points  of  exit  from  the  canal.  In  the  dorsal  region, 
however,  it  will  be  observed  that  each  nerve  is  slightly  inclined  back- 
wards from  the  side  of  the  cord  to  the  foramen  by  which  it  emerges.  In 
the  lumbar  region   this  backward   inclination  of  the  nerves  becomes 


Portion  of  Spinai.  Cord  with  the  Roots  of  the  Nerves  {QiuCin). 

1.  Inferior  median  fissure  ;   2.  Superior  median  fissure ;   3.   Infero-lateral  fissure    (exaggerated) 

4.  Supero-lateral  fissure  ;  5.  Inferior  roots,  passing  under  the  ganglion  (on  the  left  side  these  are  cut) 
6.  Superior  roots,  the  fibres  of  which  pass  into  the  ganglion — C  ;  7.  The  united  or  compound  nerve 

5.  Superior  primary  branch  ;  9.  Inferior  primary  branch. 

augmented,  and  it  continues  to  increase  in  the  same  way  in  each  nerve 
of  the  sacral  and  coccygeal  regions.  The  sacral  nerves  thus  have  their 
roots  detached  from  the  lumbar  part  of  the  cord,  while  the  coccygeal 
nerves  are  given  off  by  the  tei-minal  part  of  the  cord,  which,  as  already 


140  THE  ANATOMY  OF  THE  HORSE. 

stated,  does  not  extend  be3-ond  the  middle  of  the  sacrum.  These  last 
nerves  have  therefore  a  length  of  several  inches  within  the  spinal  canal ; 
and  as  they  pass  back  together,  each  to  reach  its  aperture  of  exit,  they 
have  an  arrangement  which  resembles  the  hairs  of  a  horse's  tail,  and  is 
therefore  termed  the  cauda  equina. 

KooTS  of  the  Nerves.  Each  spinal  nerve  has  two  roots  connecting 
it  with  the  spinal  cord — a  superior  and  an  inferior.  The  superior, 
sensory,  or  ganglionic  root  consists  of  filaments  which  arise  from  along 
the  supero-lateral  fissure  of  the  cord.  These  filaments  perforate  the 
dura  mater,  and  converge  towards  the  intervertebral  foramen,  where 
they  form  a  cord  on  which  there  is  superposed  a  reddish  oval  ganglion. 
Immediately  beyond  the  ganglion  the  cord  mixes  its  fibres  with  the 
inferior  root. 

The  inferior,  motw\  or  aganglionic  root  consists  of  fibres  detached 
from  the  cord  along  its  infero-lateral  fissure.  These,  which  are  fewer 
and  smaller  than  those  of  the  superior  root,  perforate  the  dura  mater 
by  openings  distinct  from  those  for  the  superior  root ;  and,  converging 
towards  the  intervertebral  foramen,  they  join  the  superior  root  immedi- 
ately external  to  the  point  at  which  the  ganglion  is  placed  on  it.  The 
fibres  of  the  inferior  root,  thus,  have  no  connection  with  the  ganglion. 
Where  the  superior  and  inferior  roots  meet  in  the  intervertebral  foramen, 
they  mix  their  fibres  and  form  a  short  common  cord,  which  almost 
immediately  divides  into  two — the  superior  and  the  inferior  primary 
branch;  and  each  of  these  contains  fibres  from  both  roots.  Both 
branches  emerge  by  an  intervertebral  foramen,  and,  roughly  speaking, 
the  series  of  superior  primary  branches  supply  the  skin  and  muscles 
above  their  points  of  emergence,  while  the  inferior  primary  branches  are 
distributed  to  the  skin  and  muscles  below  their  points  of  emergence, 
including  the  limbs.  From  the  common  trunk  formed  by  the  union  of 
the  two  roots,  a  filament  re-enters  the  spinal  canal  to  be  distributed  to 
the  bones  and  vessels. 

In  the  region  of  the  neck  the  spinal  accessm'y  nerve  (page  255)  passes 
along  each  side  of  the  cord,  between  the  superior  and  inferior  roots  of 
the  spinal  nerves.  It  is  formed  by  rootlets  that  spring  out  of  the  side 
of  the  cord. 

The  Vessels  of  the  Spinal  Cord. 

The  Middle  Spinal  Artery  begins  beneath  the  cord,  in  the  ring  of 
the  atlas.  It  is  here  formed  on  the  mesial  plane  by  the  fusion  of  right 
and  left  branches,  each  of  which  is  the  posterior  branch  formed 
by  the  bifurcation  of  the  cerebro-spinal  artery.  The  middle  spinal 
artery  passes  backwards  beneath  the  inferior  median  fissure  of  the  cord. 
Its  branches  are  distributed  to  the  cord  and  its  membranes.  As  it  passes 
backwards  giving  off  its  branches,  it  is  reinforced  by  other  arteries 
entering  at  the  intervertebral  foramina.     Thus,  at  each  intervertebral 


DISSECTION    OF    THE    BACK    AND    THORAX. 


141 


foramen  iu  the  neck  a  branch  of  the  vertebral  artery  enters  the  spinal 
canal.  In  the  back  similar  branches  enter  from  the  dorso-spinal  division 
of  the  intercostal  arteries,  in  the  loins  the  branches  emanate  from  the 
lumbar  arteries,  and  in  the  sacral  region  from  the  lateral  sacral  artery. 
As  a  rule,  the  branches  entering  by  adjacent  foramina  anastomose,  and 
then  give  off  branches  to  the  cord  and  the  vertebral  bodies. 

The  Veins  of  the  Spinal  Cord  are  tortuous,  and  form  on  its  surface 
a  plexus  from  which  the  blood  passes  into  two  large  veins  that  lie  one  at 
each  side  of  the  superior  common  hgament.  These  receive  also  veins 
from  the  vertebral  bodies,  and  they  are  drained  by  vessels  that  issue  by 
the  intervertebral  foramina  to  join  the  vertebral,  intercostal,  lumbar,  or 
lateral  sacral  veins. 

Directions. — For  the  examination  of  the  structure  of  the  spinal  cord, 
a  few  inches  of  it  with  the  roots  of  the  nerves  intact  should  be  procured, 
and  kept  in  spirit  or  some  other  hardening  fluid  for  at  least  a  week. 
A  portion  from  the  spinal  cord  of  any  of  the  domestic  animals  will  serve 
the  purpose  ;  but,  from  its  larger  size,  that  of  the  horse  is  to  be  pre- 
ferred. 

Structure  of  the  Spinal  Cord.  The  meninges  having  been  removed, 
the  student  will  note  the  following  points  regarding  the  surface  of  the 
cord  : — It  approaches  the 
cylindrical  in  form,  but  is 
slightly  flattened  above  and 
below.  It  is  traversed  in 
the  longitudinal  direction 
by  three  fissures,  and  a 
fourth  is  sometimes  de 
scribed.  The  supeHofr  med 
ian  fissure  is  a  narro^\ 
interval  extending  into  the 
cord  along  the  middle  line 
of  its  upper  face.  It  is 
occupied  by  neuroglia.  The 
inferior  median  fissure  \^  an 
actual  cleft  penetrating  the 
cord  along  the  middle  line 
of  its  lower  face.  It  is 
occupied  by  a  process  of 
pia  mater.  The  supero- 
lateral fissure  is  a  faint 
surface  depression  extend- 
ing on  the  side  of  the  cord, 
along  the  line  of  emergence 
of  the  superior  roots  of  the  spinal  nerves. 


Fig.  17. 

Teansverse  Section  of  Spinal  Cord  of  Calf  {Klein). 

1.  Superior  median  fissure  ;  2.  Central  canal,  in  grey 
(superior)  commissure  ;  3.  Superior  horn  of  grey  matter  ;  4. 
Inferior  horn  of  grey  matter;  5.  Process  of  pia  mater  in 
inferior  median  fissure  ;  6.  White  (inferior)  commissure  ; 
7.  Inferior  column  of  white  matter  ;  8.  Inferior  nerve  roots ; 
9.  Lateral  column  of  white  matter;  10.  Pia  mater;  11. 
Superior  column  of  white  matter  ;  12.  Superior  nerve  roots. 


The  infero-lateral  fissure  has 


142  THE  ANATOMY  OF  THE  HORSE. 

no  actual  existence,  but  is  sometimes  described  as  extending  along  the 
line  of  emergence  of  the  inferior  roots. 

These  fissures  will  be  better  seen  in  a  transverse  section  of  the  cord. 
This  should  be  made  with  a  sharp  scalpel,  so  as  to  leave  a  clean-cut 
surface.  On  examining  this  surface,  the  superior  median  fissure  will  be 
seen  to  extend  inwards  to  near  the  centre  of  the  cord,  while  the  inferior 
median  fissure,  which  is  wider  but  not  so  deep,  also  extends  towards 
the  centre  of  the  cord.  The  superior  and  inferior  median  fissures  do 
not  quite  meet,  being  separated  by  a  bridge  of  tissue  connecting  the 
right  and  left  halves  of  the  cord.  This  bridge  of  tissue  is  made  up  of 
the  grey  and  white  commissures  of  the  cord.  The  grey  commissure  stretches 
across  the  bottom  of  the  superior  median  fissure,  and  in  its  centre  there 
will  be  seen  a  dot-like  mark,  which  is  the  section  of  the  central  canal  of 
the  spinal  cord.  This  canal  extends  throughout  the  whole  length  of  the 
spinal  cord;  and  where  the  cord  joins  the  brain,  the  canal  is  continued 
into  the  medulla  oblongata,  in  which  it  opens  into  the  4th  ventricle. 
The  white  commissure  forms  a  thinner  stratum  than  the  preceding, 
beneatli  which  it  stretches  at  the  top  of  the  inferior  median  fissure. 

It  will  be  observed  that  in  each  half  of  the  cord  there  are  two  kinds 
of  nerve  tissue,  distinguished  by  a  difference  in  colour. 

1.  There  is  the  grey  matter,  which  lies  in  the  interior,  and  has  a 
crescentic  form.  The  convex  side  of  each  crescent  is  turned  inwards, 
and  the  right  and  left  crescents  are  connected  by  the  grey  commissure. 
The  extremities  of  the  crescent  ai-e  tei-med  its  horns.  The  superior  horn 
is  acute,  and  is  prolonged  to  the  supero-lateral  fissure  by  a  single 
bundle  of  fibres  belonging  to  a  superior  nerve-root.  The  inferior  horn 
is  rounded,  and  lies  some  distance  beneath  the  surface  of  the  cord. 
From  it  several  bundles  of  fibres  pass  to  form  an  infei-ior  nerve-root. 
The  grey  matter  of  the  cord  contains  nerve  cells,  medullated  and  non- 
medullated  nerve  fibres,  and  delicate  nerve  fibrillse.  The  nerve  cells 
are  mostly  of  the  multipolar  variety,  and  are  most  numerous  in  the 
inferior  horn.  The  connective-tissue  of  the  cord,  both  here  and  in  the 
white  matter,  is  a  delicate  substance  termed  neuroglia. 

2.  The  white  matter  in  each  half  of  the  cord  surrounds  the  crescent, 
and  it  is  divided  into  three  columns  by  the  crescent  and  the  nerve 
bundles  passing  from  the  horns.  The  superior  column  lies  between  the 
superior  median  fissure  and  the  upper  half  of  the  crescent.  The  inferior 
column  is  included  between  the  inferior  median  fissure  and  the  lower  half 
of  the  crescent.  The  lateral  column  lies  in  the  concavity  of  the  crescent, 
its  limits  being  marked  at  the  surface  of  the  cord  by  the  supero- 
lateral fissure  and  the  line  of  emergeiice  of  the  inferior  nerve-roots. 
The  white  matter  of  the  cord  is  composed,  besides  neuroglia,  of  medul- 
lated nerve  fibres  having  for  the  most  part  a  longitudinal  direction. 

{For  the  muscles  of  this  chapter,  see  the  table  at  jjage  336.) 


CHAPTER    IV. 

DISSECTION   OF   THE   HEAD   AND   NECK. 

THE  UNDER  PART  OF  THE  NECK. 

As  the  first  stage  in  the  examination  of  this  region,  the  student  should 
dissect  the  structures  placed  below  the  cervical  vertebrae — in  other  words, 
the  under  part  of  the  neck. 

Surface-marking. — A  well-marked  groove  extends  in  the  longitudinal 
direction  on  the  side  of  this  region,  beginning  at  the  upper  part  of  the 
neck,  and  terminating  between  the  shoulder  and  the  anterior  part  of  the 
pectoral  region.  It  lodges  the  jugular  vein,  and  is  therefore  termed  the 
jugular  channel  or  furrow.  In  performing  phlebotomy  on  this  vein, 
pressure  is  made  on  the  furrow  with  the  fingers,  in  order  to  arrest  the 
downward  current  of  blood,  and  thus  distend  and  make  prominent  the 
vessel  above  the  point  of  pressure.  In  the  lower  third  of  the  fm-row 
the  vein  lies  in  company  with  the  carotid  artery,  and  it  is  in  this  situa- 
tion that  the  latter  vessel  may  be  most  conveniently  exposed  for  ligature 
or  incision.  The  boundaries  of  the  groove  will  be  learnt  after  removal 
of  the  skin. 

Position. — The  dissection  of  this  part  of  the  neck  should  be  completed 
while  the  dissector  of  the  fore  limb  is  engaged  with  the  pectoral  region, 
the  animal  being  placed  on  the  middle  line  of  its  back,  and  the  head 
being  forcibly  extended  on  the  atlas  in  order  to  put  on  the  stretch  the 
muscles  and  other  structures  to  be  dissected. 

Directions. — An  incision  through  the  skin  should  be  carried  along  the 
middle  line,  from  the  cariniform  cartilage  at  the  lower  part  of  the  neck 
to  the  centre  of  the  intermaxillary  space.  From  the  latter  point  a 
curved  incision  should  be  carried  outwards  a  little  behind  the  angle  of 
the  jaw,  as  far  as  the  wing  of  the  atlas.  These,  in  conjunction  Avith  the 
incisions  made  by  the  dissector  of  the  pectoral  region,  will  permit  the 
skin  to  be  reflected  upwards  as  far  as  the  middle  of  the  side  of  the  neck. 
The  cutaneous  nerves  and  the  cervical  panniculus  should  then  be 
examined. 

Cutaneous  Nerves  of  the  neck.  Five  stellate  groups  of  nei-ves  will 
be  seen  perforating  the  mastoido-humeralis  muscle.  These  are  derived 
from  the  inferior  primary  branches  of  the  cervical  spinal  nerves  from  the 


144  THE  AN  ATOM  V  OF  THE  HORSE. 

2nd  to  the  6th.  The  first  of  these  appears  behind  the  wing  of  the 
ath^s ;  and,  besides  twigs  to  the  upper  part  of  the  neck,  it  sends  into 
the  intermaxillary  space  a  branch  which  may  be  traced  to  near  the 
symphysis,  and  auricultir  branches  which  will  subsequently  be  followed 
to  the  skin  of  the  ear.  Some  branches  from  the  lowest  group  turn  dov,na- 
wards  and  backwards  over  the  mastoido-humeralis  in  front  of  the  shoulder, 
and  spread  over  the  anterior  part  of  the  pectoral  region  (Plate  1).  The 
branches  of  the  other  groups  are  disjDosed  upwards,  downwards,  and  later- 
ally, to  supply  the  skin  of  the  neck. 

Cervical  Panniculus  {Platysma  myoides  of  man).  This  is  the  repre- 
sentative in  the  neck  of  the  muscle  which  is  much  more  strongly 
developed  in  connection  with  the  skin  over  the  trunk  and  shoulder. 
It  may  be  said  to  take  origin  at  the  lower  part  of  the  neck,  where  its 
fibres  are  fixed  to  the  cariniform  cartilage  (Plate  27).  At  this  point  it 
is  a  band  of  considerable  thickness ;  but  as  it  passes  up  the  neck,  it 
widens  and  becomes  thinner.  At  the  upper  part  of  the  neck  its  fibres 
do  not  form  a  complete  layer,  but  are  scattered  in  an  aponeurosis  which 
prolongs  the  muscle  into  the  intermaxillary  space  and  over  the  face. 
Along  the  middle  line  it  is  joined  by  means  of  a  fibrous  raphe  to  the 
muscle  of  the  opposite  side.  The  outer  edge  of  the  muscle  is  continued 
by  an  aponeurosis  over  the  mastoido-humeralis,  splenius,  and  trapezius 
muscles.  In  the  lower  half  of  the  neck  the  muscle  is  intimately 
adherent  to  the  inferior  edge  of  the  mastoido-humeralis,  and  a  careful 
dissection  is  necessary  to  separate  them.  It  covers  the  jugular  furrow, 
and  the  stemo-maxillaris,  sterno-thyro-hyoideus,  and  subscapulo-hyoideus 
muscles.  It  is  supplied  by  the  cervical  branch  of  the  7th  cranial  nerve, 
which  should  be  found  entering  it  at  the  upper  part  of  the  jugular 
fun-ow,  and  running  on  the  deep  face  of  the  muscle  or  in  its  substance 
where  it  covers  the  furrow. 

Action. — The  cervical  panniculus,  unlike  the  panniculus  of  the  trunk, 
is  but  slightly  adherent  to  the  skin,  which,  therefore,  it  can  twitch  only 
slightly.  Its  principal  action  seems  to  be  to  brace  the  muscles  over 
Avhich  it  is  spread,  and  by  its  adhesion  to  the  mastoido-humeralis  it 
may  aid  in  depressing  the  neck.    ' 

Directions. — Beginning  at  the  middle  line  of  the  neck,  the  dissector 
should  carefully  remove  the  foregoing  muscle.  This  will  expose  the 
juoTilar  furrow  lodging  the  jugular  vein.  After  that  vessel  has  been 
examined,  a  little  dissection  will  serve  to  separate  the  muscles  in 
relation  to  the  trachea. 

The  Jugular  Vein  (Plate  27)  is  the  large  vessel  which  drains  away 
the  blood  from  the  head  and  the  upper  part  of  the  neck.  It  is  formed 
by  the  junction  of  the  superficial  temporal  and  internal  maxillary  veins, 
which  unite  at  the  deep  face  of  the  parotid  gland,  below  and  behind  the 
temporo-maxillary  articulation.     It  passes  outwards  through  the  parotid, 


DISSECTION    OP    THE    HEAD    AND    NECK.  145 

and  then  lies  in  a  groove  on  the  surface  of  the  gland ;  but  this  part  of 
its  course  is  not  to  be  examined  at  present.  At  the  lower  extremity  of 
the  parotid  it  is  joined  by  a  large  branch — the  submaxillary  vein  ;  and 
it  then  passes  into  the  jugular  furrow,  in  which  it  descends  to  the 
entrance  to  the  chest.  The  upper  boundary  of  the  furrow,  it  will  now 
be  observed,  is  formed  by  the  mastoido-humeralis,  and  the  lower  by  the 
sterno-maxillaris.  In  the  upper  half  of  this  groove  the  vein  rests  on  the 
subscapulo-hyoideus  muscle,  which  there  separates  the  vessel  from  the 
carotid  artery ;  but  in  the  lower  half  the  vein  rests  on  the  side  of  the 
trachea,  and  is  in  direct  contact  with  the  carotid,  which  lies  above  and 
slightly  internal  to  it.  The  jugular  of  the  left  side  differs  from  the 
right  in  being  related,  in  the  lower  part  of  the  groove,  to  the  oesophagus 
as  well  as  the  trachea.  At  the  entrance  to  the  chest  the  right  and  left 
jugulars  unite  with  one  another  and  with  the  axillary  veins,  thus  forming 
the  initial  portion  of  the  anterior  vena  cava. 

The  jugular  receives,  in  the  part  of  its  course  now  exposed,  the 
following  branches : — 

1.  The  Suhviaxillary  or  Facial  vein,  which  joins  the  jugular  at  an 
acute  angle  in  which  lies  the  inferior  extremity  of  the  parotid  gland. 

2.  The  Thyroid  vein,  bringing  blood  from  the  thyroid  body  and 
larynx. 

3.  Innominate  cutaneous,  muscular,  oesophageal,  and  tracheal  branches, 
whose  disposition  is  not  constant. 

4.  The  Cephalic  vein,  which  enters  the  jugular  near  its  termination. 

[The  single  jugular  of  the  horse  is  generally  said  to  be  the  representative  of  the  external 
jugular  of  man  ;  the  internal  jugular,  under  that  view,  being  undeveloi^ed  in  the  soliped. 
This  I  believe  to  be  a  mistake,  and  for  the  following  reasons.  The  external  jugular  of 
man  runs  on  the  surface  of  the  platysma  (panniculus),  and  never  beneath  it  as  does  the 
vein  of  the  horse  ;  moreover,  it  is  a  vessel  of  very  variable  volume,  being  frequently  small, 
and  sometimes  absent.  On  the  other  hand,  tlie  cervical  part  of  the  internal  jugiilar  of 
man  has  a  situation  exactly  corresponding  to  that  of  the  horse's  vein,  save  that  the  latter 
vessel  is  generally  superficially  placed  to  the  subscapulo-hyoid  (omo-hyoid) ;  and  in  the 
liorse  I  have  seen  the  jugular,  otherwise  normal,  pass  under  that  muscle,  keeping  com- 
j)any  with  the  carotid  artery  for  the  whole  of  its  coiuse.] 

The  Sterno-maxillaris  (Plate  27).  This  muscle  corresponds  to  the 
inner  portion  of  the  sterno-mastoid  of  man.  It  arises  from  the  carini- 
form  cartilage  of  the  sternum,  and  is  inserted  by  a  flat  tendon  into  the 
angle  of  the  inferior  maxilla.  In  the  lower  half  of  the  neck  the  muscle 
lies  below  the  trachea,  and  covers  the  sterno-thyro-hyoideus  muscle. 
In  this  position  the  right  and  left  muscles  are  in  contact,  but  about  the 
middle  of  the  neck  they  diverge,  and  cross  obliquely  upwards  and 
forwards  over  the  trachea  and  the  subscapulo-hyoideus  muscle.  Its 
terminal  tendon  is  included  between  the  parotid  and  submaxillary  glands. 
The  upper  edge  of  the  muscle  forms  the  lower  boundary  of  the  jugular 
furrow.  In  its  lower  part  the  muscle  is  thick  and  rounded,  but  it 
becomes  more  slender  and  flattened  as  it  is  traced  upwards. 

L 


146  THK  ANATOMY  OF  THE  HORSK. 

Action. — To  depress  (flex)  the  head  or  give  it  a  hxteral  incliuatiou, 
according  as  the  right  and  left  muscles  act  singly  or  in  concert. 

The  Sterno-thyro-hyoideus  (Plates  27  and  28).  This  is  a  long  and 
slender  nniscle,  extending  along  the  lower  face  of  the  trachea,  and  closely 
applied  along  the  middle  line  to  its  fellow  of  the  opposite  side.  It  takes 
origm  from  the  carinifurm  cartilage  of  the  sternum.  About  the  middle 
of  the  neck  its  muscular  substance  is  interrupted  by  a  short  tendinous 
portion,  rendering  the  muscle  digastric.  Above  this  central  tendon  the 
muscle  divides  into  two  portions.  The  outer  or  thyroid  band  passes 
obliquely  outwards  and  forwards  between  the  trachea  and  the  sub- 
scapulo-hyoideus  muscle,  and  becomes  inserted  into  the  edge  of  the 
thyroid  cartilage  of  the  larynx.  The  inner  or  hyoid  band  is  continued 
directly  forwards  in  company  with  the  corresponding  branch  of  the 
opposite  imiscle,  and  becomes  inserted  into  the  body  of  the  hyoid  bone. 

Action  — To  depress  the  hyoid  bone  and  larynx. 

The  SuBSCAPULO-HYOiDEUS  {Omo-hyoid  of  man)  (Plates  27  and  28). 
This  is  a  thin,  ribbon-shaped  muscle  having  a  breadth  of  three  or  four 
inches.  It  takes  origin  at  the  inner  side  of  the  scapula,  from  the  fascia 
covering  the  subscapularis  muscle.  It  then  passes  downwards  and  for- 
wards between  the  scalenus  and  rectus  capitus  anticus  major  muscles 
inwardly;  and  the  supraspinatus,  anterior  deep  pectoral,  and  mastoido- 
humeralis  muscles  outwardly.  Appearing  at  the  lower  edge  of  the 
last-named  muscle,  to  which  it  adheres  closely,  it  passes  between  the 
jugular  vein  and  carotid  artery ;  and  crossing  over  the  upper  part  of  the 
trachea  in  a  dii'ectiou  obliquely  forwards  and  downwards,  it  applies  itself 
at  the  outer  edge  of  the  hyoid  band  of  the  sterno-thyi-o-hyoideus,  and 
becomes  inserted  along  with  that  muscle  into  the  body  of  the  hyoid 
bone.  In  the  lower  part  of  the  neck  the  ascending  branch  of  the 
inferior  cervical  artery  and  the  prescapular  group  of  lymphatic  glands, 
are  included  between  this  muscle  and  the  mastoido-humcralis. 

Action. — To  depress  the  hyoid  bone. 

Nerves.  At  the  upper  part  of  the  neck  a  branch  from  the  spinal 
accessory  nerve  enters  the  sterno-maxillaris,  awd  branches  from  the  1st 
spinal  nerve  enter  the  sterno-thyro-hyoid  and  subscapulo-hyoid  muscles. 
These,  however,  will  be  better  dissected  at  a  later  stage. 

Directions. — The  jugular  vein  should  now  be  ligatured  at  tlie  upper 
and  lower  ends  of  the  jugiilar  furrow,  and  the  intermediate  portion  of 
the  vessel  should  be  cut  away.  The  excised  portion  of  the  vein  should 
be  laid  open  to  expose  its  valves.  The  part  of  the  subscapulo-hyoideus 
which  passes  over  the  trachea  may  be  cut  out  after  the  manner  of 
Plate  28,  and  the  sterno-maxillaris  may  be  similarly  treated.  This  will 
expose  for  examination  the  trachea,  the  oesophagus,  the  carotid  artery, 
the  pneumogastric  and  sympathetic  nerves,  and  the  recui-rent  nerve. 

Valves  of  Veins.     Three  or  four  valves  are  placed  in  the  jugular  vein. 


DISSECTION    OP    THE    HEAD    AND    NECK.  147 

Each  valve  is  composed  of  two  or  three  seraihinar  folds  of  the  inner  coat 
of  the  vein,  the  folds  having  a  close  resemblance  to  the  semilunar  seg- 
ments of  the  aortic  valve  (Fig.  11,  page  129).  Each  flap  with  the  adjacent 
part  of  the  wall  of  the  vein  forms  a  small  pouch  with  its  mouth  directed 
towards  the  heart.  When  the  blood  tends  to  regurgitate,  it  distends 
these  pouches  until  the  segments  meet  across  the  vessel  and  thus  arrest 
the  backward  current.  In  most  veins  throughout  the  body  similar 
valves  are  found;  but  they  are  most  numerous  in  the  veins  of  the  limbs. 
In  the  small  veins  each  valve  may  be  composed  of  only  a  single  flap. 
The  following  veins  have  few  or  no  valves : — the  pulmonary  system  of 
veins,  the  veins  of  the  portal  system,  the  hepatic  veins,  the  anterior  and 
posterior  vense  cavse,  and  the  veins  of  the  brain. 

The  Trachea,  or  wind-pipe  (Plate  28),  begins  beneath  the  altanto-axial 
articulation,  where  it  is  continuous  with  the  larynx.  It  descends  in  the 
middle  plane  of  the  neck,  beneath  the  spinal  column  ;  and  passing 
between  the  first  two  ribs,  it  gains  the  thorax,  where  it  bifurcates  to 
form  the  bronchi.  In  the  neck  the  muscles  of  the  region  envelop  the 
trachea,  and  are  related  to  it  as  follows  : — The  longus  colli  is  related  to 
its  upper  aspect,  the  sterno-thyro-hyoideus  extends  along  its  lower  face, 
the  sterno-maxillaris  crosses  its  direction  obliquely  upwards  and  forwards, 
the  subscapulo-hyoideus  crosses  it  obliquely  downwards  and  forwards, 
and  at  the  lower  part  of  the  neck  it  contacts  on  each  side  with  the 
scalenus.  It  is  also  related  to  the  oesophagus,  the  carotid  artery,  the 
jugular  vein,  and  the  pneumogastric,  sympathetic,  and  recurrent  nerves. 

The  Thyroid  Body  or  Gland  (Plate  29)  is  related  to  the  upper  part 
of  the  trachea  on  each  side,  resting  on  its  first  four  rings.  The  gland 
has  a  rounded  form,  and  a  reddish-brown  colour;  and  it  is  richly 
supplied  with  blood,  which  it  receives  from  the  thyroid  and  thyro- 
laryngeal  branches  of  the  carotid  artery.  Sometimes  a  narrow  inthmns 
connects  the  right  and  left  glands  across  the  lower  face  of  the  trachea. 
The  gland  has  an  investing  capsule  of  fibrous  connective-tissue,  continuous 
with  an  internal  trabecular  framework.  Under  the  microscope  the  sub- 
stance of  the  organ  is  seen  to  contain  numerous  spherical  spaces,  each  lined 
by  a  single  layer  of  epithelium,  and  filled  by  a  viscid  colloid  material. 

The  (Esophagus,  or  gullet  (Plate  28),  is  a  segment  of  the  alimentary 
canal.  It  begins  above  the  larynx,  Avhere  it  is  continuous  with  the 
pharynx.  It  descends  on  the  upper  face  of  the  trachea,  and  in  the  first 
few  inches  of  its  course  it  lies  in  the  middle  plane  of  the  neck,  being 
related  to  the  longus  colli  muscle  above.  It  soon,  however,  begins  to 
deviate  to  the  left  side,  so  that  below  the  middle  of  the  neck  it  lies 
rather  on  the  upper  part  of  the  left  side  of  the  wind-pipe.  Maintaining 
this  relationship,  the  two  tubes  enter  the  thorax  in  company,  the  gullet 
being  prolonged  through  that  cavity  to  pass  by  the  foramen  sinis- 
trum  of  the  diaphragm  into  the  abdomen,  where  it  terminates  in  the 


148  THE  ANATOMY  OK  THE  HORSE. 

stomach.  The  gullet  is  related  to  the  muscles  of  the  left  side  already 
enumerated  as  contacting  with  the  trachea,  the  stcrno-thyro-hyoideus 
excepted.  It  is  also  related  to  the  carotid  artery,  the  jugular  vein,  and  the 
pneumogastric,  sympathetic,  and  recurrent  nerves  of  the  left  side.  The 
cervical  part  of  the  ousoj^hagus  has  the  external  appeai*ance  of  a  volun- 
tary muscle,  for  which  it  is  often  mistaken  at  first  sight  by  the  student. 
The  examination  of  its  structure,  as  well  as  that  of  the  trachea,  must 
be  postponed  until  the  accompanying  vessels  and  nerves  have  been 
examined. 

The  Common  Carotid  Artery  (Plate  28).  This  is  the  vessel  that 
conveys  the  blood  to  the  head  and  upper  part  of  the  neck.  It  begins 
on  the  under  aspect  of  the  trachea,  at  the  entrance  to  the  thorax,  where 
it  results  from  the  bifurcation  of  a  short  vessel  termed  the  cephalic 
trunk — a  branch  of  the  arteria  innominata.  It  ascends  in  the  neck  in 
company  with  the  trachea,  and  terminates  above  the  cricoid  cartilage  of 
the  larynx  by  dividing  into  the  external  carotid,  the  internal  carotid, 
and  the  occipital  artery.  It  thus  crosses  the  trachea  very  obliquely, 
being  at  first  on  its  iinder  surface,  then  on  its  lateral  aspect,  and  finally 
above  it.  It  is  in  contact  with  the  scalenus,  longus  colli,  rectus 
capitis  anticus  major,  and  subscapulo-hyoideus  muscles,  the  last- 
mentioned  intervening  between  the  artery  and  the  jugular  vein  in  the 
upper  half  of  the  neck.  In  the  lower  half  of  the  neck  the  artery  and 
vein  are  in  direct  contact,  the  carotid  being  above  and  slightly  internal 
to  the  jugular.  The  common  cord  of  the  pneumogastric  and  sympathetic 
nerves  is  on  the  upper  side  of  the  artery,  and  the  inferior  laryngeal 
(recurrent)  nerve  is  below  it.  At  the  entrance  to  the  thorax  the  pre- 
pectoral  group  of  lymphatic  glands  is  in  contact  with  the  artery.  The 
left  carotid  difl:ers  from  the  right  in  being  related  for  a  considerable 
part  of  its  course  to  the  ojsophagus,  which  separates  it  from  the  trachea. 
The  collateral  branches  of  the  carotid  are  as  follows  : — 

1.  Innominate  and  slender  imiscular,  oesophageal,  and  </-ac/iea/ branches. 

2.  The  Thyroid  artery,  which  arises  a  few  inches  behind  the  thyroid 
body,  and  passes  obliquely  forwards  to  enter  the  gland  on  its  posterior 
aspect.  Sometimes  this  artery  is  distributed  mainly  or  entirely  to  the 
neighbouring  muscles. 

3.  The  Thyro-laryngeal  artery. — This  is  the  largest  of  its  collateral 
branches.  It  arises  a  little  in  front  of  the  preceding  vessel,  and  passing 
to  the  inner  side  of  the  thyroid  body,  it  divides  in  front  of  it  into 
thyroid  and  laryngeal  branches.  The  former  turn  back  to  enter  the 
gland  in  front,  while  the  latter  pass  to  the  larynx  and  pharynx. 

The  terminal  branches  of  the  carotid  are  not  to  be  followed  at 
present. 

The  Pneumogastric  and  Sympathetic  Nerves  in  the  neck  (Plate  28). 
The  pneumogastric,  vagus,  or  10th  cranial  nerve  has  its  origin  from 


DISSECTION    OF    THE    HEAD    AND    NECK.  149 

the  medulla  oblongata.  It  leaves  the  cranium  by  the  posterior  part  of 
the  foramen  lacerum,  and  inclining  downwards  and  backwards  on  the 
guttural  pouch,  it  meets  the  cervical  cord  of  the  sympathetic,  with 
which  it  becomes  in  nearly  all  cases  closely  united.  The  common  cord 
resulting  from  the  fusion  of  the  two  nerves  descends  in  company  with 
the  carotid  artery,  lying  on  the  upper  side  of  that  vessel.  At  the  lower 
pai't  of  the  neck  the  two  nerves,  in  passing  into  the  thoi-ax,  again 
become  separate. 

The  cervical  cord  of  the  sympathetic  begins  at  the  superior  cervical 
ganglion,  which  rests  on  the  guttural  pouch.  After  a  short  course  it 
unites,  as  just  described,  with  the  vagus. 

No  branches  are  given  off  from  either  the  vagus  or  the  sympathetic  in 
the  part  where  they  form  a  common  cord. 

The  Inferior  Laryngeal  (Recurrent)  Nerve  (Plate  28).  This 
is  a  branch  of  the  vagus,  given  off"  within  the  thorax.  The  right  nerve 
has  its  origin  in  front  of  the  heart,  and  is  reflected  round  the  dorso- 
cervical  artery.  The  left  nerve  is  longer  than  the  right,  having  its 
point  of  detachment  at  the  base  of  the  heart,  where  it  is  reflected  round 
the  root  of  the  posterior  aorta.  The  nerves  pass  forwards  on  the  trachea, 
and  enter  the  neck  by  passing  between  the  first  pair  of  ribs.  In  the 
neck  each  nerve  ascends  below  the  carotid  artery,  the  right  nerve 
resting  on  the  trachea,  but  that  of  the  left  side  being,  for  the  greater 
part  of  its  course,  on  the  oesophagus.  The  nerves  will  subsequently 
be  followed  in  their  distribution  to  the  larynx.  In  the  neck  each 
recurrent  nerv^e  throws  off"  branches  to  the  trachea  and  oesophagus. 

Directions. — At  this  stage  the  dissector  of  the  fore  limb  will  be 
engaged  with  the  dissection  of  the  axilla,  and  the  dissector  of  the  neck 
should  co-operate  with  him  in  the  examination  of  the  mode  of  formation 
of  the  brachial  plexus  of  nerves,  and,  thereafter,  of  the  levator  anguli 
scapulee  muscle.  For  the  brachial  plexus  turn  to  page  3,  and  for  the 
levator  anguli  scapulae  to  page  8. 

Prepectoral  Lymphatic  Glands.  This  is  a  large  group  of  glands 
placed  beneath  and  at  the  side  of  the  great  vessels  at  the  entrance  to 
the  chest.  They  are  placed  on  the  course  of  the  lymphatic  vessels  of 
the  head,  neck,  and  fore  limb. 

Directions. — A  segment  about  six  inches  in  length  may  now  be  cut 
from  the  trachea,  and  a  similar  segment  from  the  oesophagus.  These 
are  to  be  dissected  to  display  the  structure  of  the  two  tubes. 

Structure  of  the  Trachea.  This  comprises  (1)  a  framework  of 
cartilages  united  by  (2)  fibro-elastic  membrane ;  (3)  an  incomplete 
layer  of  non-striped  muscular  tissue  ;  (4)  a  submucous  layer  ;  and  (5) 
a  mucous  lining. 

The  Cartilages  of  the  trachea  number  between  fifty  and  sixty,  and 
are  of  the  hyaline  variety.     Although  usually  denominated  the  rings  of 


150  THE  ANATOMY  OF  THE  HORSE. 

the  trachea,  they  do  not  form  complete  circles,  but  have  rather  a 
resemblance  to  the  letter  C  with  its  ends  overlapping.  In  consequence 
of  this  configuration  of  the  rings,  the  trachea  is  not  circular  on  section, 
but  flattened  in  the  vertical  direction ;  and  the  overlapping  of  the 
extremities  of  the  cartilages  takes  place  on  the  middle  of  the  upper 
aspect  of  the  tube.  The  breadth  of  the  rings  is  not  quite  uniform,  but 
averages  about  half  an  inch.  Here  and  there,  however,  two  adjacent 
rings  may  be  more  or  less  fused  by  the  obliteration  of  the  uniting 
fibro-elastic  membrane.  In  the  thoracic  portion  of  the  tube  the 
extremities  of  the  rings  do  not  meet,  and  the  deficiency  is  there  made 
up  by  a  number  of  thin  cartilaginous  pieces  of  irregular  size  and  shape, 
and  somewhat  imbricated  in  their  arrangement.  The  rings  are  thickest 
and  strongest  in  their  central  portion,  and  thinnest  at  their  extremities. 
The  Fibro-elastic  Membrane. — This  connects  the  adjacent  edges  of  the 
cartilages,  and  at  the  upper  wall  of  the  tube  it  connects  their  overlapping 
extremities.  Its  extensibility  and  elasticity  permit  the  length  of  the 
trachea  to  be  accommodated  to  the  movements  of  the  neck,  and  these 
properties  will  be  made  very  evident  by  alternately  extending  and 
relaxing  a  segment  of  the  tube  containing  five  or  six  rings. 

The  Trachealis  3h<sde. — This  is  a  layer  of  non-striped  muscular  tissue 
having  its  fibres  directed  transversely.  It  does  not  extend  all  round 
the  tube,  but  is  confined  to  its  upper  part,  where  the  fibres  lie  internal 
to  the  extremities  of  the  rings  or  the  fibro-elastic  membrane.  The 
fibres  form  a  continuous  band,  being  not  only  placed  under  each  ring, 
but  also  in  the  interval  between  adjacent  rings. 

The  Subimicous  Coat  is  composed  of  areolar  connective-tissue  with 
numerous  elastic  fibres  longitudinally  disposed.  It  also  contains  many 
small  compound  racemose  glands,  whose  mucous  secretion  is  discharged 
by  ducts  opening  on  the  free  surface  of  the  mucous  membrane. 

The  Mucous  Membrane,  which  forms  a  complete  internal  lining  to 
the  tube,  possesses  a  stratified  epithelium,  the  surface  layer  of  cells  lieing 
ciliated. 

Structure  of  the  CEsophagus.  This  comprises  (1)  a  muscular  coat, 
arranged  in  two  layers ;  (2)  a  submucous  coat;  a:id  (3)  a  mucous  lining. 
The  Muscular  Coat  consists  of  (ct)  an  outer  layer  of  fibres  longi- 
tudinally disposed,  and  (b)  a  deeper  layer  in  which  the  fibres  are 
arranged  as  transverse  or  oblique  rings.  In  the  cervical  part  of  the  tube, 
and  in  the  thoracic  part  about  as  far  as  tlie  heart,  the  nniscular  fibres 
are  for  the  most  part  of  the  striped  variety,  and  the  tube  has  there 
the  external  appearance  of  a  voluntary  muscle.  About  the  centre  of 
the  thorax,  however,  the  character  of  the  fibres  gradually  changes  to 
the  pale,  non-striped  variety  of  muscular  tissue,  and  l^chind  that  point 
the  tube  is  therefore  pale  like  the  stomach  or  the  intestines. 

The  Submucous  Coat  is  composed  of  areolar  connective-tissue  contain- 


DISSECTION    OF    THE    HEAD    AND    NECK.  151 

ing  the  alveoli  of  numerous  mucous  glands,  whose  ducts  penetrate  the 
mucous  membrane.  It  forms  a  very  loose  bond  of  connection  between 
the  muscular  and  mucous  layers ;  and  when  the  oesophagus  is  cut 
across,  the  mucous  coat  appears  almost  as  if  it  lay  independently 
within  the  muscular  layer. 

The  Mucous  Membrane  has  a  thick  stratified  epithelium  ;  and,  except 
during  the  act  of  deglutition,  its  free  surface  is  thrown  into  longitudinal 
folds  which  meet  with  one  another  and  obliterate  the  lumen  of  the  tube. 
In  colour  it  is  whitish,  owing  to  its  low  vascularity  and  the  thickness 
of  its  epithelial  covering. 

THE  UPPER  PART  OF  THE  NECK. 

Position. — The  animal  should  be  suspended  in  imitation  of  the  natural 
standing  posture,  in  the  manner  described  at  page  8,  for  the  dissection 
of  the  outer  scapular  region. 

Directions.— The  whole  of  the  neck  behind  the  atlas  should  be 
denuded  of  skin.  The  cutaneous  nerves  of  the  region  should  then  be 
noticed,  and  the  spinal  accessory  nerve  should  be  found  crossing  obliquely 
backwards  and  upwards  on  the  surface  of  the  splenius  muscle. 

Cutaneous  Nerves.  For  the  most  part,  the  cutaneous  nerves  of  this 
region  are  dei'ived  from  the  stellate  groups  already  seen  perforating  the 
mastoido-humeralis.  These  are  derived  from  the  inferior  primary 
branches  of  the  cervical  spinal  nerves  from  the  2nd  to  the  6th.  Other 
branches,  which  are  derived  from  the  superior  primary  branches  of  the 
same  nerves,  emerge  near  the  middle  line  of  the  neck  above,  and  are 
distributed  to  the  integument  beneath  the  mane. 

The  Spinal  Accessory  (11th  Cranial)  Nerve  (Plate  27).  This 
nerve  derives  its  fibres  from  the  medulla  oblongata  and  the  cervical 
part  of  the  spinal  cord.  It  leaves  the  cranium  by  the  foramen  lacerum 
basis  cranii,  passes  backwards  on  the  guttural  pouch,  turns  upwards 
over  the  edge  of  the  wing  of  the  atlas,  and  passes  obliquely  backwards 
and  upwards  beneath  the  mastoido-hiimeralis  muscle.  Appearing  at 
the  upper  edge  of  the  last-named  muscle,  it  is  continued  in  the  same 
direction  on  the  surface  of  the  splenius,  and  disappears  beneath  the 
cervical  trapezius,  in  which  and  the  dorsal  trapezius  it  terminates. 
While  the  neck  is  elevated,  the  trunk  of  the  nerve  is  thrown  into 
numerous  short  sinuosities,  apparently  to  obviate  stretching  of  the 
nerve  when  the  neck  is  depressed. 

Dii-ections. — The  cervical  portions  of  the  trapezius  and  rhomboideus 
muscles  should  now  be  examined  in  co-operation  with  the  dissector  of  the 
fore  limb ;  and,  thereafter,  the  mastoido-humeralis  is  to  be  dissected. 

The  Trapezius.     See  page  9. 

The  Rhomboideus.     See  page  10. 

The  Mastoido-Humeralis,  or  Levator  Humeri  (Plates  27  and  28). 


152  THK    AX  ATOMY    OF    THE    HOR>!E. 

This  is  a  long  and  powerful  umscle,  extending  between  the  head  and  the 
shoulder,  on  the  side  of  the  spinal  column.  It  takes  oriffin  from  the 
mastoid  process  and  crest,  from  the  wing  of  the  atlas,  and  from  the 
transverse  processes  of  the  2nd,  3rd,  and  4th  cervical  vertebra).  The 
tendon  of  origin  from  the  mastoid  process  and  crest,  which  is  not  to  be 
exposed  at  present,  is  thin  and  aponeurotic ;  that  from  the  wing  of  the 
atlas  is  common  to  the  splenius  and  trachelo-mastoideus  muscles ;  while 
the  succeeding  slips  of  origin  are  fleshy.  The  muscle  passes  over  the 
shoulder-joint,  and  becomes  inserted  into  the  outer  lip  of  the  musculo- 
spiral  groove.  As  already  seen,  the  lower  edge  of  the  muscle  forms  the 
upper  boundary  of  the  jugular  channel,  and  at  the  lower  part  of  the 
neck  it  is  closely  united  to  the  sternal  band  of  the  panniculus. 

Action. — It  is  an  extensor  and  inward -rot  at  or  of  the  shoulder-joint. 
When  the  limb  is  fixed,  it  bends  the  neck  laterally. 

This  muscle  represents  the  greater  part  of  the  sterno-mastoid  of  man 
(the  rest  being  represented  by  the  sterno-maxillaris),  combined  with  the 
clavicular  part  of  the  deltoid,  this  fusion  resulting  from  the  absence  of 
a  clavicle. 

Directions. — If  the  mastoido-hTmieralis  has  not  already  been  cut,  it 
should  be  divided  in  front  of  the  shoulder,  and  turned  upwards  (Plate 
28)  to  show  the  prescapular  glands  and  a  branch  of  the  inferior 
cervical  artery.  The  stellate  groups  of  cutaneous  nerves  may  thereafter 
be  traced  through  the  mastoido-humei-alis  to  their  source. 

The  Prescapular  Lymphatic  Glands.  These  are  arranged  in  the 
form  of  a  chain  between  the  mastoido-humeralis  and  subscapulo-hyoid 
muscles  at  the  lower  part  of  the  neck. 

The  Inferior  Cervical  Artery  is  a  branch  of  the  axillary,  arising 
at  the  first  rib.  It  divides  into  a  descending  (Plate  1)  and  an 
ascending  branch,  the  latter  being  here  seen  between  the  mastoido- 
humeralis  and  subscapulo-hyoid  muscles,  to  which  and  the  above- 
mentioned  glands  it  is  distributed. 

Cervical  Spinal  Nerves.  There  are  eight  pairs  of  these,  the  1st  issuing 
from  the  spinal  canal  by  the  antero-intenial  foramen  of  the  atlas,  the 
2nd  by  the  foramen  (converted  notch)  at  the  anterior  edge  of  the  arch 
of  the  axis,  and  the  others  in  succession  by  the  intei'vertebral  foramina. 
They  have  all  a  common  disposition  in  that  each  divides  at  its  point  of 
exit  into  superior  and  inferior  primary  branches.  Only  the  inferior 
primary  branches  present  themselves  now  for  consideration,  and  of  these 
the  1st  is  more  conveniently  taken  at  a  later  stage.  The  remaining  six 
behave  as  follows  : — 

The  2nd,  3rd,  4th,  5th,  and  6th  communicate,  each  with  the  preced- 
ing and  succeeding  branches  of  the  series,  and  divide  into  three  sets  of 
branches,  viz.,  (1)  communicating  brandies  to  the  middle  cervical  ganglion 
(see  vertebral  nerve,  page  157);  (2)  muscular  branches  to  the  mastoido- 


DISSECTION    OF    THE    HEAD    AND    NECK.  153 

humeralis,  longus  colli,  scalenus,  and  rectus  capitis  anticus  major 
muscles,  and  to  the  diaphragm  (see  phrenic  nerve,  page  6);  (3)  cutaneous 
branches  which  pierce  the  mastoido-humeralis  and  are  distributed  as 
the  stellate  groups  already  seen.  Besides  these,  the  6th  nerve  sends 
branches  to  the  levator  anguli  scapulae  and  rhomboideus  muscles,  and 
its  phrenic  branch  sends  a  twig  to  the  brachial  plexus. 

The  branches  of  the  7th  and  8th  nerves  are  expended  in  the  brachial 
plexus  after  each  has  detached  a  communicating  filament  to  the  middle 
cervical  ganglion,  that  from  the  7th  joining  the  vertebral  nerve,  while 
that  from  the  8th  passes  to  the  ganglion  independently. 

Directions. — The  dissector  of  the  fore  limb  will  now  be  in  a  position 
to  separate  the  limb  from  the  trunk,  which  will  permit  the  dissection  of 
the  remainder  of  the  neck.  The  levator  anguli  scapula;,  as  left  by  the 
dissector  of  the  fore  limb,  must  now  be  entirely  removed  in  order  to 
expose  the  posterior  part  of  the  splenius.  The  mastoido-humeralis  may 
also  be  cut  away  as  far  forwards  as  tlie  vertebra  dentata.  The  inser- 
tions of  the  splenius,  trachelo-mastoideus,  and  complexus  muscles  into 
the  head  are  not  to  be  exposed  at  present,  as  that  would  involve  the 
destruction  of  the  muscles  of  the  ear  and  other  structures  not  yet 
dissected. 

The  Splenius  (Plate  27).  This  is  a  flat,  fleshy  muscle  of  a 
triangular  form,  having  its  fibres  directed  downwards  and  forwards.  It 
takes  origin  from  the  2nd,  3rd,  and  4th  dorsal  spines,  and  from  the 
funicular  portion  of  the  ligamentum  nuchee.  Its  origin  from  the  dorsal 
spines  is  aponeurotic,  and  confounded  with  that  of  the  anterior  serratus 
and  complexus  muscles.  It  is  inserted  into  the  mastoid  crest,  the 
wing  of  the  atlas,  and  the  transverse  processes  of  the  succeeding  four 
cervical  vertebrae.  The  mastoid  insertion  is  flat  and  aponeurotic,  and 
is  united  to  the  mastoid  tendon  of  the  trachelo-mastoideus.  The 
insertion  into  the  atlas  is  tendinous  and  riband-like,  and  is  common  to 
the  trachelo-mastoideus  and  mastoido-humeralis.  The  other  insertions 
are  fleshy. 

Action. — The  right  and  left  muscles  acting  together  elevate  the 
head  and  neck  ;  acting  singly,  they  incline  the  head  and  neck  to  the 
side  of  the  acting  muscle. 

Directions. — The  origin  of  the  splenius  should  be  carefully  detached, 
and  the  muscle  should  be  raised  and  turned  downwards  so  as  to  expose 
the  subjacent  structures.  Nerves  from  the  superior  primary  branches 
of  the  cervical  nerves,  and  branches  from  the  superior  cervical,  dorsal, 
and  vertebral  arteries,  will  be  foimd  entering  its  deep  face.  The 
trachelo-mastoideus  and  complexus  miiscles,  now  exposed,  should  be 
dissected,  the  branches  of  nerves  and  vessels  found  in  connection  with 
them  being  as  far  as  possible  preserved. 

The  Trachelo-Mastoideus.      This   muscle   consists   of  two  parallel 


154  THE  ANATOMY  OF  THE  HORSE. 

fleshy  portions  extending  along  the  spine,  under  cover  of  the  splenins, 
and  resting  on  the  coniplexus.  The  fibres  of  the  muscle  arise  by 
successive  slips  from  the  transverse  processes  of  the  first  two  dorsal 
vertebrae ;  and,  in  common  with  the  complexiis,  from  the  articular 
processes  of  the  last  six  cervical  vertcbrfe.  The  upper  division  of  the 
muscle  is  inserted,  by  a  flat  tendon  common  to  the  splenins,  into  the 
mastoid  crest ;  the  lower  division  terminates  in  a  riband-like  tendon, 
common  to  the  splenius  and  mastoido-humeralis,  and  inserted  into  the 
wing  of  the  atlas. 

Action. — Acting  singly,  to  bend  tlie  neck  laterally ;  acting  with 
the  opposite  muscle,  to  extend  the  occipito-atlantal  articulation  (elevate 
tlie  head). 

The  CoMPLEXUS.  This  is  one  of  the  most  powcrt'ul  muscles  of  the 
neck.  It  covers  the  lamellar  portion  of  the  ligamentum  michte,  which 
separates  the  right  and  left  muscles.  It  arises  from  the  2nd,  3rd,  and 
4th  dorsal  spines,  in  common  with  the  splenius;  from  the  transverse 
processes  of  the  first  six  or  seven  dorsal  vertebrae;  and  from  the  articular 
processes  of  the  cervical  vertebrae  with  the  exception  of  the  first.  From 
these  different  points  of  origin  the  fibres  converge  towards  the  poll,  where 
they  terminate  in  a  tendon  inserted  into  the  occipital  bone. 

Action. — Both  muscles  will  extend  the  occipito-atlantal  joint  (elevate 
the  head) ;  the  muscle  of  one  side  will,  while  elevating  the  head,  turn 
it  slightly  to  the  same  side. 

Nerves.  As  already  stated,  eacli  cervical  nerve  resolves  itself  into  a 
superior  and  an  inferior  primary  branch.  The  siipcrior  primary  branches 
of  the  last  six  may  now  be  foiind  distributing  nerves  to  the  splenius, 
ti-achelo-mastoideus,  complexns,  and  semispinalis  colli  muscles ;  and  if 
the  complexns  be  raised  from  the  ligamentum  nuchte,  other  branches 
will  be  found  to  ascend  between  the  muscle  and  the  ligament  to  be 
distributed  to  the  integument  near  the  middle  line. 

The  muscles  and  other  structures  which  lie  above  the  cervical 
vertebrae  receive  their  chief  blood  supply  from  three  vessels,  viz.,  the 
occipital,  dorsal,  and  superior  cervical  arteries.  The  first  of  these  gives 
branches  to  the  neighboui-hood  of  the  poll,  but  it  is  not  to  be  sought  at 
present. 

The  Dorsal  Artery  will  be  found  distributing  branches  to  the  upper 
part  of  the  neck,  in  front  of  the  withers.  The  artery  has  its  origin 
within  the  thorax,  where,  on  the  left  side,  it  is  a  branch  of  the  axillary 
artery,  and,  on  the  right  side,  of  the  arteria  innominata.  It  leaves  the 
chest  by  the  upper  part  of  the  second  intercostal  space ;  and  inclining 
backwards  and  upwards,  it  divides  on  the  longissimus  dorsi  into  a 
number  of  branches  distributed  to  the  withers,  and  the  neck  in  front  of 
that  region.  These  will  be  foiuid  ascending  between  the  splenius  and 
complexns  miiscles. 


DISSECTION    OF    THE    HEAD    AND    NECK.  155 

The  Superior  Cervical  Artery,  like  the  preceding,  arises  within  the 
chest,  and  from  the  same  source.  It  leaves  the  thorax  by  the  upper 
part  of  the  first  intercostal  space  ;  and  placing  itself  on  the  inner 
surface  of  the  complexus,  it  ascends  between  that  muscle  and  the  liga- 
mentum  nuchse,  as  fiir  as  the  2nd  or  3rd  cervical  vertebra,  where  its 
terminal  branches  anastomose  with  those  of  the  occipito-muscular  and 
dorsal  arteries. 

Veins.  The  dorsal  and  superior  cervical  arteries  are  accompanied 
by  veins  of  the  same  names,  Avhich,  after  entering  the  chest,  discharge 
themselves  into  the  anterior  vena  cava. 

Directions. — The  complexus,  splenius,  and  trachelo-mastoideus  muscles 
may  now  be  removed  as  far  as  the  hinder  end  of  the  axis.  This  will 
expose  the  semispinalis  colli  muscle  and  the  ligamentum  nuchse. 

The  Semispinalis  Colli  Muscle  (Fig.  18)  rests  on  the  laminae  of 
the  cervical  vertebrse,  and  consists  of  five  bundles.  The  most  posterior 
of  these  bundles  may  be  described  as  taking  origin  from  the  anterior 
articular  process  of  the  7th  cervical  vertebra,  and  passing  forwards 
and  inwards  to  be  inserted  into  the  superior  spine  of  the  6th  vertebra. 
The  most  anterior  bimdle  passes  in  the  same  way  between  the  3rd 
vertebra  and  the  axis,  while  the  intermediate  bundles  have  corresponding- 
attachments. 

Action. — The  right  and  left  muscles,  acting  in  concert,  will  extend 
(elevate)  the  cervical  part  of  the  spinal  colunm.  The  muscle  of  one  side, 
acting  singly,  will  rotate  and  incline  the  spinal  column  to  the  opposite  side. 

The  Intertransversales  Colli  Muscles  (Fig.  18).  These  form  a 
set  of  six  muscular  b\indles  with  strong  tendinous  intersections,  and 
cover  the  sides  of  the  cervical  vertebrse.  There  is  one  bundle  for  each 
intervertebral  articulation  except  the  first.  Each  muscular  bundle 
consists  of  an  upper  and  a  lower  slip ;  and  it  may  be  described  as  arising 
from  the  articiilar  process  of  one  vertebra,  and  passing  forwards  to  be 
inserted  into  the  transverse  process  of  the  vertebra  in  front.  The 
muscles  conceal  the  intervertebral  foramina  and  the  vertebral  vessels  ; 
and  they  are  perforated  by  branches  of  these  vessels,  and  by  the  superior 
and  inferior  primary  branches  of  the  spinal  nerves  of  the  neck. 

Action. — To  bend  the  neck  laterally. 

The  Rectus  Capitis  Anticus  Major  (Plate  28  and  Fig.  18).  This 
muscle  begins  by  a  tapering  point  on  the  side  of  the  vertebral  column 
at  the  5th  cervical  vertebra  ;  and  passing  forwards  and  inwards,  it  gains 
the  inferior  face  of  the  atlas,  in  passing  to  the  base  of  the  skull.  It 
arises  from  the  transverse  processes  of  the  5th,  4th,  and  3rd  vertebrse, 
the  slip  of  origin  from  the  first  of  these  crossing  the  point  of  the 
scalenus.  Its  insertion,  which  is  not  to  be  exposed  at  present,  is  into 
the  tubercular  processes  at  the  junction  of  the  basilar  process  Avith  the 
body  of  the  sphenoid. 


156 


THE  ANATOMY  OF  THE  HORSE. 


Action. — The  riglit  and  left  muscles,  acting  together,  flex  the  head. 
When  only  one  muscle  acts,  it  inclines  the  head  to  the  same  side. 

The  Scalenus  (Plate  3  and  Fig.  18).  This  muscle  is  situated  on  the 
side  of  the  lower  half  of  the  neck.     It  arises  from  the  transverse  processes 


Fig.  18. 

LlC.AMENTUM    NUCH^   AND   DEEP   MUSCLES   OF  THE   NeCK   (C/« 


1.  Lamellar  portion  of  the  ligamentum  nnchse ;  2.  Funicular  portion  of  the  same  ;  3.  Semispinalis 
of  the  back  and  loius  ;  4.  Semispinalis  colli ;  5.  Rectus  capitis  postiois  major  ;  6.  Rectus  capitis 
posticus  minor ;  7.  Obliquus  capitis  inferior  ;  S.  Obliquus  cajiitis  siiperior  ;  0.  Intertransversales 
colli ;  10.  Rectus  capitis  anticus  major  ;  11.  External  intercostal*  ;  12.  Upper  and  lower  divisions  of 
the  scalenus. 

of  the  last  four  cervical  vei-tebra3.  In  front  of  the  1st  rib  it  is  perforated 
by  the  roots  of  the  brachial  plexus,  which  there  divide  it  into  an  upper 
and  a  lower  portion.  The  first  of  these  is  much  the  smaller  of  the  two, 
and  it  is  inserted  into  the  outer  surface  of  the  1st  rib  near  its  upper  end. 
The  lower  portion  is  inserted  into  the  anterior  border  and  outer  surface 
of  the  same  rib,  the  lowest  fibres  being  immediately  above  the  smooth 
impression  left  on  the  anterior  border  of  the  bone  by  the  axillary  vessels. 

Action. — To  pull  forwards  or  fix  the  1st  rib,  and  thus  to  aid  in 
inspiration.  When  the  rib  becomes  the  fixed  point,  the  muscles  will 
flex  the  neck  or  incline  it  to  the  side,  according  as  the  right  and  left 
muscles  act  in  concert  or  singly. 

The  LoNGUs  Colli  (Plate  28).  This  muscle  clothes  the  inferior  face 
of  the  spinal  column  from  the  6th  dorsal  vertebra  to  the  atlas,  the 
right  and  left  muscles  being  closely  united  along  the  middle  line,  while 
at  its  outer  edge  each  muscle  is  partially  blended  with  the  intertrans- 


DISSECTION    OF    THE    HEAD    AND    NECK.  157 

verse  muscles.  The  dorsal  portion  of  the  muscle  is  seen  in  the 
dissection  of  the  thorax  (Plates  22  and  25),  where  its  fibres  take  origin 
from  the  bodies  of  the  first  six  dorsal  vertebrae,  and  pass  forwards  to 
terminate  in  a  tendon  inserted  into  the  6th  cervical  vertebra.  In  the 
neck  the  fibres  of  tlie  muscle  take  origin  from  the  transverse  processes, 
and  each  bundle  passes  with  a  forward  and  inward  direction,  to  be 
inserted  into  the  body  of  a  vertebra  anterior  to  that  from  which  it  arises. 
The  most  anterior  ftisciculi  terminate  in  a  tendon  inserted  into  the 
tubercle  of  the  athvs. 

Action. — To  bend  the  neck  downwards. 

Directions. — The  vertebral  vessels  and  the  accompanying  nerve  should 
now  be  exposed  by  the  careful  removal  of  the  intertransversales  muscles, 
attention  being  at  the  same  time  directed  to  the  superior  and  inferior 
primary  branches  of  the  spinal  nerves.  These  nerves  emerge  in 
common  from  the  intervertebral  foramina,  but  separately  pierce  the 
muscles. 

The  Vertebral  Artery  is  a  branch  of  the  axillary  artery,  given  off 
from  that  vessel  before  it  leaves  the  thorax.     It   enters  the  neck  by  '^'*^,'Asi 
passing  forwards  to  the  inner  side  of  the  1st  rib  a  little  below  its  upper 
end.      It  then   ascends   along  the  side   of  the  spinal  column,   passing- 
first  beneath  the  transverse  process  of  the  7th  vertebra,  and  then  in 
succession  through  the  vertebral  foramina  of  the  other  bones  as  far  as 
the   axis.     Between   the   last-mentioned   bone   and   the   atlas   it  joins 
directly  the  retrograde  branch  of  the  occipital  artery,  but  this  is  not  to 
be  exposed  at  present.     It  throws  off  in   its  course  (1)  muscidar  and 
(2)  spinal  branches.     The  former  are  very  numerous  and  consist  of  an      '^J^ 
upward,  a  downward,  and  an  outward  set.     Many  of  the  upward  set         r^ 
cross  over  the  vertebrae,  and  anastomose  with  branches  from  the  superior 
cervical  artery.     The  spinal  branches  are  detached  from  the  itftier  si(Je 
of  the   artery;  and   entering   the    spinal    canal  by   the    intervertebJ 
foramina,  they  join  the  middle  spinal  artery  in  supplying  the  spinal  cof 
and  its  coverings. 

The  Vertebral  Vein  accompanies  the  artery,  and  within  the  chest 
joins  the  anterior  vena  cava. 

The  Vertebral  Nerve  runs  in  close  company  with  the  vessels.  It 
is  formed  by  the  union  of  filaments  from  the  inferior  primary  branches 
of  the  cervical  nerves  from  the  2nd  to  the  7th.  In  the  thorax  it  joins 
the  inferior  cervical  ganglion  of  the  sympathetic  nerve.  It  is  thus 
a  composite  nerve  made  up  of  the  afferent  filaments  sent  by  the  before- 
mentioned  spinal  nerves  to  the  sympathetic  cord. 

The  Ligamentum  Nuch^  (Fig.  18).  This  is  the  largest  ligament  in 
the  body.  It  is  placed  on  the  middle  plane  of  the  neck,  above  the 
vertebrae,  and  it  consists  of  a  right  and  a  left  division,  each  of  which, 
again,  comprises  a  funicular  and  a  lamellar  portion.      The  entire  liga- 


158  THE  ANATOMY  OK  THE  HORSE. 

ment,  like  most  of  the  other  ligaments  of  the  neck,  is  composed  of 
yellow  elastic  tissue. 

The  funicular  portion  has  the  form  of  a  flattened  cord  united  by  its 
inner  edge  to  the  corresponding  structure  of  the  opposite  side.  Poster- 
iorly, behind  the  summit  of  the  3rd  dorsal  spine,  this  cord  is  continuous 
with  the  supraspinous  ligament  of  the  back.  Anteriorly  the  cord  is 
inserted  into  a  special  tubercle  on  the  occipital  bone.  Between  these 
points  of  attachments  the  cord  extends  with  a  slight  upward  concavity 
when  the  ligament  is  relaxed,  and  above  it  there  is  developed,  in  vary- 
ing amount,  a  quantity  of  fatty-elastic  tissue  supporting  the  integument 
from  which  the  mane  grows. 

The  lamellar  portion  is  triangular  in  form,  occupying  the  interval 
between  the  funicular  portion  and  the  vertebral  column.  Its  fibres 
have  a  downward  and  forward  direction,  being  fixed  above  to  the  funi- 
cular portion  or  to  the  spines  of  the  2nd  and  3rd  dorsal  vertebrae,  and 
below  to  the  spines  of  the  last  six  cervical  vertebrae.  The  fibres  are 
stronger  and  more  closely  aggregated  in  proportion  as  they  are  anterior, 
the  lamella  forming  a  complete  septum  in  its  anterior  two-thirds,  but 
having  the  form  of  a  network  in  its  posterior  third.  The  right  and  left 
lamellae  are  applied  together  on  the  mesial  plane,  their  inner  fiices  being 
united  by  areolar  connective-tissue. 

The  ligamentum  nuchte  assists  in  suspending  the  head  ;  and  when 
the  head  has  been  depressed,  it  aids  the  muscles  in  elevating  it  again. 
But  for  its  presence,  a  large  additional  amount  of  muscular  tissue 
would  have  been  necessary  in  the  neck.  In  man,  in  whom  the  head  is 
supported  by  the  spinal  column,  the  ligament  is  very  rudimentary,  and 
has  lost  its  elastic  texture. 

Directions. — The  spinal  column  should  now  be  disarticulated  between 
the  3rd  and  4th  cervical  vertebrre ;  and  the  head  should  be  laid  aside 
on  a  clean  table,  while  the  student  proceeds  to  the  dissection  of  the 
remaining  ligaments  of  the  neck.  It  will  suffice  to  dissect  cai-efully  the 
ligaments  of  one  intervertebral  ai-ticulation, — say  that  between  the  4th 
and  5tli  bones. 

The  intervertebral  joints  of  the  neck  posterior  to  the  vertebra  dentata 
are  constructed  after  a  common  plan,  which  is  also  that  of  the  dorsal 
and  lumbar  regions.  Each  vertebra  is  articulated  to  the  preceding  and 
the  succeeding  bone  (1)  by  an  amphiarthrodial  union  of  the  bodies,  and 
(2)  by  diarthrodial  joints  between  the  articular  processes. 

The  atlanto-axial  and  the  occipito  atlantal  joints,  which  are  purely 
diarthrodial  articulations,  will  be  dissected  at  a  later  stage. 

LIGAMENTS    AND    ARTICULATIONS    OF    THE    NECK    POSTERIOR    TO 
THE    DENTATA. 

The  ligaments  may  be  classified  into  (1)  those  connecting  the  pro- 


DISSECTION    OF    THE    HEAD    AND    NECK.  159 

cesses  and  neural  arches,  and  (2)  those  connecting  the  adjacent  vertebral 
bodies. 

Ligaments  of  the  Processes  and  Neural  Arches  : — 

The  Liff amentum  Nuchae. — This  has  already  been  dissected. 

The  Intersjnnous  Ligaments. — These  are  composed  of  yellow  elastic 
tissue.  Each  consists  of  two  narroAV  parallel  bands  stretching  between 
adjacent  superior  spinous  processes. 

Capsular  Ligaments  of  the  articular-  processes. 

Ligamenta  suhjlava. — For  these  two  series  of  ligaments,  see  page  135, 
where  the  coi-responding  ligaments  of  the  back  and  loins  are  described. 
In  the  neck  these  ligaments  difter  from  those  of  the  other  regions  in 
being  composed  of  yellow  elastic  tissue. 

Ligaments  of  the  Bodies  : — 

The  Suj)erior  Common  Ligament  lies  on  the  floor  of  the  spinal  canal, 
and  terminates  in  front  at  the  axis.     See  page  135. 

The  Intervertehral  Substance. — See  page  136. 

Movements  of  the  cervical  part  of  the  spinal  column.  These  are 
flexion,  extension,  lateral  inclination,  rotation,  and  circumduction,  the  last 
being  a  combination  of  the  first  three.  In  flexion  the  vertebrae  are 
carried  downwards  in  a  vertical  plane,  and  extension  is  the  opposite 
movement.  Rotation  is  the  twisting,  or  turning,  of  a  vertebra  round  a 
longitudinal  axis  passing  through  its  body.  In  consequence  of  the 
thickness  of  the  intervertebral  substance,  and  the  feeble  development  of 
the  transverse  and  spinous  processes,  all  these  movements  have  here  a 
greater  range  than  in  the  back  or  loins  ;  and  Avithin  the  cervical  x-egion 
the  greatest  range  of  movement  is  permitted  in  the  posterior  joints. 

tub  external  ear  (fig.  19), 

The  organ  of  hearing  consists  of  three  divisions :  the  external,  the 
middle,  and  the  internal  ear.  Only  the  first  of  these  will  now  be 
examined.  The  middle  and  the  internal  ear,  Avhich  are  cavities  within 
the  petrous  temporal  bone,  ai-e  described  at  page  267,  The  external  ear 
comprises  the  external  auditory  process  of  the  petrous  temporal  bone ; 
three  cartilages — conchal,  scutiform,  and  annular ;  muscles  which  move 
these  cartilages  ;  vessels  ;  and  nerves. 

Directions. — An  incision  through  the  skin  is  to  be  begun  a  few  inches 
behind  the  summit  of  the  occipital  bone,  and  carried  down  the  middle 
line  as  far  as  the  supraorbital  process.  It  is  here  to  be  carried  outwards 
along  the  supraorbital  process,  and  then  backwards  along  the  zygomatic 
arch.  On  reaching  the  articulation  of  the  jaw,  the  incision  should  be 
carried  along  the  edge  of  the  vertical  ramus,  and  inwards  to  the  middle 
line.  All  the  skin  mapped  out  by  this  incision  is  to  be  removed,  the 
conchal  cartilage  being  also  denuded  of  its  outer  covering.  This  will 
expose  not  only  the  parts  of  the  external  ear,  but  also  the  parotideal 


160  THE  ANATOMY  OF  THE  HOUSE. 

region  and  the  poll,  and  the  dissection  of  these  parts  is  to  be  made  as 
soon  as  the  ear  is  finished.  On  one  side  the  mnscles  and  cartilages  of 
the  ear  may  be  dissected ;  and  then  the  other  side  may  be  denuded  of 
skin  in  the  same  manner  as  the  first,  in  order  to  follow  the  vessels  and 
nerves. 

Muscles  of  the  Ear. — These  are  divided  into  extrinsic  and  intrinsic. 
The  former  have  their  origin  from  extraneous  parts,  but  the  latter  both 
arise  from,  and  are  insei-ted  into,  the  cartilages  of  the  ear.  The  cartil- 
ages of  the  ear  cannot  be  fully  exposed  until  the  muscles  have  been 
examined,  but  it  may  be  premised  that  the  conchal  cartilage  is  the  lai-ge 
trumpet-like  cartilage  which  mainly  gives  to  the  outer  ear  its  form ; 
that  the  annular  cartilage  is  a  short  tube,  or  ring,  which  is  telescoped 
on  to  the  external  auditory  process,  and  is  itself  embraced  by  the  con- 
stricted base  of  the  concha ;  and  that  the  scutiform  cartilage  is  a  thin 
plate  which  rides  on  the  surface  of  the  temporal  muscle,  in  front  of  the 
base  of  the  concha. 

Extrinsic  Muscles. 

The  Parotido-auricularis,  or  Deprimens  Aurem  (Plate  29).  This 
muscle  has  the  form  of  a  broad  riband.  It  arises  from  the  outer 
surface  of  the  parotid  gland;  and  passing  vertically  upwards,  it  is 
inserted  into  the  outer  part  of  the  base  of  the  concha,  below  the 
opening. 

Action. — To  incline  the  ear  downwards  and  outwards. 

The  Cervico-auriculares,  or  Retrahentes  Aurem.  There  are 
three  of  these,  distinguished  as  the  cervico-auricularis  (or  retrahens) — 
externus,  medius,  and  internus.  They  all  arise  from  the  poll  in  the 
neighbourhood  of  the  insertion  of  the  ligamentura  nuchae,  and  they  are 
here  superposed  the  one  to  the  other.  Suppose  the  ear  to  be  placed 
with  the  opening  of  the  concha  looking  directly  outwards,  then  the 
externus  is  iriserted  into  the  middle  of  the  inner  face  of  the  concha ;  the 
medius  into  the  outer  side  of  the  concha,  beneath  the  opening,  and 
under  cover  of  the  parotido-auricularis  miiscle ;  and  the  internus  into 
the  base  of  the  concha,  on  its  posterior  aspect,  and  under  cover  of  the 
parotid  gland. 

Action. — In  moderate  contraction,  these  muscles  give  the  opening  of 
the  concha  an  outward  direction ;  and  when  forcibly  contracted,  they 
direct  the  opening  backwards  as  well  as  outwards,  and  incline  the  ear 
towards  the  poll. 

The  Parieto-auricularis  Externus,  or  Attolens  Maximus.  This  is 
a  wide,  membranous  muscle  covering  the  temporalis  muscle.  It  arises 
from  the  parietal  crest ;  and  it  is  inserted  by  its  upper  fibres  into  the 
scutiform  cartilage,  and  by  its  lower  into  the  front  of  the  conchal 
cartilage.  Its  upper  fibres  are,  at  their  origin,  continuous  across  the 
middle  line  with  the  opposite  muscle. 


DISSECTION    OF    THE    HEAD    AND    NECK. 


161 


Action. — To  prick  the  ear,  that  is,  to  erect  it  and  give  its  opening  a 
forward  direction. 

The  Zygomatico-auricularis,  or  Attolens  Anticus.  This  muscle  is 
continuous  with  the  preceding  by  an  intermediate  aponeurosis,  and 
its  own  muscular  substance  is  generally  divided  into  two  slips  by 
intermediate  fascia.     It  arises  from  the  zygomatic  process  of  the  squa- 


FiG.  19. 

Auricular  Muscles  and  Nerves  of  a  Mule  (CJiauveoM), 
1.  Parieto-auricularis  externus  ;  2.  Parieto-auricularis  internus  ;  3.  Scutiform  cartilage  ;  4.  Scuto- 
auricularis  externus  ;  5.  Temporalis ;  6.  Corrugator  supercilii  ;  7.  Orbicularis  palpebrarum  ;  S. 
United  tendons  of  the  levatores  labii  superioria  proprli ;  9.  Dilatator  naris  transversalis ;  A.  Auri- 
cular branches  of  Ist  cervical  nerve  ;  B.  Anterior  auricular  nerve  (of  7th) ;  C.  Supraorbital  nerve  ; 
D.  Auricular  branch  of  the  lachrymal  nerve. 


mous  temporal  bone ;  and  it  is  inserted  by  an  inner  slip  into  the  scutiform 
cartilage,  and  by  an  outer  slip  into  the  outer  aspect  of  the  base  of  the 
concha. 


162  THE  ANATOMY  OF  THE  HORSE. 

Action. — To  prick  the  ear,  like  the  preceding  muscle. 

The  Parieto-auricularis  Internus,  or  Attolens  Posticus.  This 
muscle  is  to  be  exposed  by  the  removal  of  the  parieto-auricularis  e.xter- 
nus,  beneath  the  upper  part  of  which  it  lies.  It  arises  from  the  upper 
part  of  the  parietal  crest  ;  and  it  is  inserted  into  the  inner  side  of  the 
concha,  beneath  the  cervico-auricularis  externus. 

Action. — It  is  the  opponent  of  the  parotido-auricularis,  bringing  the 
ear  into  the  erect  position. 

The  Mastoido-auricularis.  This  muscle  is  to  be  exposed  by  cutting 
the  preceding  and  the  cervico-auricular  muscles,  and  forcibly  depressing 
the  ear  outwards.  This  will  expose,  at  the  base  of  the  ear,  a  considerable 
quantity  of  fat,  which  is  constantly  present,  and  facilitates  the  move- 
ments of  the  ear.  On  clearing  away  this  fat,  the  muscle  will  be  found 
at  the  inner  side  of  the  base  of  the  ear.  It  forms  a  slender  fasciculus 
arising  from  the  auditory  process,  and  inserted  into  the  base  of  the  concha. 

Action. — To  telescope  the  conchal  on  the  anniilar  cartilage. 

Intrinsic  Muscles. 

Besides  some  scattered  fibres  on  the  outer  and  inner  surfaces  of 
the  concha,  this  group  includes,  the  following  two  distinct  muscles 
passing  between  the  conchal  and  scutiform  cartilages. 

The  ScuTO-AURicuLARis  Externus  consists  of  two  bundles  of  fibres 
passing  between  the  outer  surface  of  the  scutiform  cartilage  and  the 
inner  side  of  the  concha. 

Action. — To  assist  the  parieto-auricularis  extenius  in  pricking  the  car. 

The  ScuTO-AURicuLARis  Inteenus.  In  order  to  expose  this  muscle, 
the  scutiform  cartilage  is  to  be  raised  from  the  surface  of  the  temporal 
muscle,  and  turned  upwards  and  outwards.  The  muscle  is  stronger 
than  the  preceding,  and  consists  of  two  distinct  crossed  bundles,  which 
arise  from  the  inner  surface  of  the  scutiform  cartilage,  and  pass 
round  the  inner  side  of  the  base  of  the  concha  to  .get  inserted  into  its 
posterior  aspect,  above  and  internal  to  the  insertion  of  the  retrahens 
internus 

Action. — It  opposes  the  preceding  muscle,  and  assists  the  retrahcntes 
in  rotating  the  concha  so  as  to  turn  the  opening  outwards  and  back- 
wards. 

Vessels. 

The  ear  is  supplied  with  blood  by  the  anterior  and  posterior  auricular 
arteries. 

The  Anterior  Auricular  Artery  (Plate  28)  is  one  of  the  two  ter- 
minal branches  of  the  superficial  temporal  artery,  which  vessel  divides 
under  the  parotid  gland,  about  an  inch  below  the  condyle  of  the 
lower  jaw.  It  ascends  behind  the  capsular  ligament  of  the  jaw  ;  and 
after  detaching  muscular  and  cutaneous  branches  in  front  of  the  ear,  it 
enters  the  temporalis  muscle. 


DISSECTION    OF    THE    HEAD    AND    NECK.  163 

The  Posterior  Auricular  Artery  (Plate  28)  is  a  collateral  branch  of 
the  external  carotid,  detached  while  that  vessel  lies  over  the  great  cornu 
of  the  hyoid  bone,  and  beneath  the  parotid  gland.  It  divides  on  the 
occipito-styloid  muscle  into  an  anterior  and  a  posterior  branch.  The 
anterior  branch  ascends  in  the  parotid,  and  ramifies  on  the  concha  behind 
the  posterior  edge  of  its  opening,  after  giving  branches  to  the  base  of  the 
ear,  and  to  the  interior  of  the  concha.  The  posterior  branch  ascends  in  the 
parotid  gland,  and  crosses  behind  the  base  of  the  ear,  beneatli  the  retra- 
hentes  muscle.  It  then  passes  vmder  the  parieto-auricularis  internvis, 
and  ascends  on  the  inner  surface  of  the  concha  as  far  as  its  tip. 

The  blood  is  drained  away  from  the  ear  by  the  anterior  and  posterior 
auricular  veins. 

The  Anterior  Auricular  Vein  is  a  larger  vessel  than  the  satellite 
artery.  It  joins  the  subzygomatic  vein  to  form  the  superficial  temporal 
tnmk. 

The  Posterior  Auricular  Vein  is  formed  at  the  base  of  the  ear  by 
two  roots  which  unite  at  the  posterior  edge  of  the  parotido-auricularis 
muscle.  It  descends  at  first  on  the  surface  of  the  parotid,  and  then  in 
its  substance,  where  it  joins  the  jugular  vein. 

Nerves. 

These  are  derived  from  the  7th  cranial  nerve,  from  the  1st  and  2nd 
cervical  nerves,  and  from  the  lachrymal  nerve  of  the  trifacial. 

The  Posterior  Auricular  Nerve  is  detached  from  the  7th  cranial 
nerve  as  it  issues  from  the  stylo-mastoid  foramen.  It  ascends  beneath 
or  in  the  substance  of  the  parotid  gland,  in  company  with  the  artery  of 
the  same  name ;  and  passing  immediately  behind  the  mastoid  process, 
it  gains  the  back  of  the  ear,  and  is  distributed  to  the  cervico-auriculares 
muscles. 

The  Middle  Auricular  Nerve  is  detached  at  the  same  point  as  the 
preceding.  It  ascends  over  the  annular  cartilage,  behind  the  peaked 
process  of  the  concha,  which  it  enters  at  its  base.  It  is  here  distributed 
to  the  scattered  muscular  fibres  on  the  interior  of  the  cartilage. 

The  Anterior  Auricular  Nerve  is  much  larger  than  either  of  the 
preceding  nerves.  It  is  given  off  from  the  7th  midway  between  the 
stylo-mastoid  foramen  and  the  edge  of  the  inferior  maxilla.  It  ascends 
in  the  parotid,  turns  over  the  zygomatic  arch,  passes  downwards  be- 
neath the  parieto-auricularis  muscle,  then  internal  to  the  root  of  the 
supraorbital  process  of  the  frontal  bone,  and  terminates  below  the  nasal 
canthus  of  the  eyelids.  It  supplies  the  attolentes  muscles  as  well  as 
the  corrugator  supercilii  and  the  orbicularis  palpebrarum,  and  its  ter- 
minal filaments  enter  the  levator  labii  superioris  aleeque  nasi. 

The  Cervical  Branch  of  the  7th  nerve.  This  nerve  comes  out 
through  the  substance  of  the  parotid  gland,  near  or  at  the  same  point 
as  the  jugular  vein,  and  under  cover  of  the  parotido-auricularis  muscle. 


164  THE  ANATOMY  OF  THE  HORSE. 

It  supplies  that  muscle,  aud  is  continued  down  the  neck,  as  already 
seen  (page  144). 

Auricular  Branches  of  the  1st  Cervical  Nerve.  These,  which  are 
derived  from  the  superior  primary  branch  of  that  nerve,  appear  in  the 
poll  between  the  obliquus  capitis  superior  and  the  rectus  capitis  posti- 
cus muscles.  Crossing  the  cervico-auricularcs  muscles,  they  are  distri- 
buted at  the  inner  side  of  the  base  of  the  ear. 

Auricular  Branches  of  the  2nd  Cervical  Nerve.  These  are  derived 
from  the  stellate  group  which  the  inferior  primary  branch  of  that  nerve 
forms  on  the  mastoido-humeralis,  behind  the  wing  of  the  atlas.  They 
reach  the  ear  by  crossing  over  the  parotid  gland,  and  are  distributed 
mainly  to  the  skin  of  the  concha  on  its  posterior  aspect  (when  the 
opening  is  directed  outwards),  but  some  branches  reach  its  inner  side. 

Auricular  Branch  of  the  Lachrymal  Nerve.  This  nerve,  which 
emerges  from  the  orbital  sheath,  crosses  the  direction  of  the  anterior 
auricular  branch  of  the  7th  on  the  zygomatic  arch,  and  is  distributed 
to  the  skin  in  front  of  the  ear. 

Cartilages  of  the  Ear. 

The  Conchal  Cartilage.  This  and  the  other  cartilages  are  com- 
posed of  yellow  (elastic)  fibro-cartilage.  Although  its  name  expresses 
some  likeness  to  a  shell,  it  bears  more  resemblance  to  a  trumpet.  The 
opening  of  the  trumpet  is  somewhat  elliptical,  and  can  be  directed 
forwards,  outwards,  or  backwards.  The  margins  of  the  opening  meet 
above  and  below  in  acute  angles.  Beneath  the  lower  angle,  or  commis- 
sure, the  cartilage  forms  a  complete  tube,  which  is  slightly  inflated  in 
form.  At  its  termination  it  becomes  nan-ow,  and  slightly  embraces  the 
annular  cartilage,  over  the  outer  side  of  which  it  sends  a  peaked  pro- 
cess, whose  fibrous  extremity  is  attached  to  the  wall  of  the  guttural 
pouch. 

The  Annular  Cartilage.  This  has  the  fonu  of  a  ring  surrounding 
the  edge  of  the  auditory  process ;  and  it  is  itself  embraced  by  the  con- 
chal cartilage,  the  three  structures  being  related  to  one  another  like  the 
tubes  of  a  telescope.  This  connection  between  the  cartilages  is  main- 
tained by  connecting  elastic  tissue,  and  by  the  lining  membrane  of  the 
ear  in  passing  from  the  one  structure  to  the  other. 

The  Scutiform  Cartilage  is  superposed  to  the  temporal  muscle  in 
front  of  the  base  of  the  concha,  to  which  it  is  connected  only  by  the 
muscles  already  described.  It  is  thin,  flexible,  and  in-egularly  triangular 
in  shape. 

the  parotideal  region. 

The  Parotid  Gland  (Plates  27,  29,  and  30).  This  is  the  largest  of  the 
salivary  glands.  It  derives  its  name  from  its  proximity  to  the  ear,  below 
the  root  of  which  it  is  placed.     From  that  point  it  stretches  downwards, 


DISSECTION    OF   THE   HEAD   AND    NECK.  165 

filling  up  the  space  between  the  wing  of  the  atlas  and  the  edge  of  the 
vertical  ramus  of  the  inferior  maxilla.  The  outer  surface  of  the  gland  is 
flat,  and  is  separated  from  the  skin  by  the  parotido-auricularis  muscle, 
and  by  the  continuation  of  the  cervical  panniculus,  which  here  takes 
the  form  of  an  aponeurosis  with  scattered  muscular  bundles.  The 
jugular  vein  lies  in  a  depression  on  the  lower  half  of  this  surface,  after 
having  become  superficial  by  passing  through  the  substance  of  the 
gland.  Below  and  behind  the  ear  the  posterior  auricular  vein  is  visible 
for  some  distance  before  it  passes  into  the  gland  to  join  the  jugular. 
Finally,  the  outer  surface  is  crossed  obliquely  upwards  and  forwards  by 
the  auricular  branches  of  the  2nd  cervical  nerve,  and  obliquely  down- 
wards and  backwards  by  the  cervical  filament  of  the  7th  cranial  nei-ve, 
Avhich  comes  out  through  the  gland  at  the  same  point  as  the  jugular, 
and  descends  under  cover  of  the  parotido-auricularis  and  panniculus 
muscles  to  be  continued  along  the  jugular  channel  of  the  neck. 
The  anterior  edge  of  the  gland  is  related  to  the  border  of  the  vertical 
ramus,  which  it  overlaps  slightly.  This  edge  is  most  intimately 
adherent  to  the  bone  and  to  the  masseter  muscle,  and  at  it  the  facial 
branches  of  the  7th  and  5th  cranial  nerves,  and  the  transverse  facial  and 
maxillo-muscular  vessels  pass  on  to  the  face  by  emerging  between  the 
gland  and  the  bone,  or  by  perforating  the  edge  of  the  former.  The  pos- 
terior edge  of  the  gland  is  related  to  the  edge  of  the  wing  of  the  atlas 
covered  by  the  mastoido-humeralis  muscle,  and  the  connection  between 
them  is  merely  by  loose  areolar  tissue.  The  upper  extremity  of  the  gland 
is  notched  to  embrace  the  root  of  the  ear,  and  beneath  or  through  it  the 
auricular  nerves  and  arteries  pass  to  the  ear.  The  inferior  extremity  of 
the  gland  is  margined  by  the  submaxillary  vein,  which  joins  the  jugular 
beneath  the  postero-inferior  angle  of  the  gland.  The  deep  face  of  the 
gland  has  numerous  and  important  relationships,  which  w^ill  be  exposed 
by  the  removal  of  the  gland ;  but  its  duct  must  first  be  examined. 

Stenson's  Duct.  This  is  formed  as  a  single  duct  by  the  union  of 
secondary  branches  at  the  anterior  edge  of  the  gland,  a  little  above  its 
lower  extremity.  It  crosses  over  the  tendon  of  the  sterno-maxillaris 
muscle,  and  enters  the  intermaxillary  space,  where  it  will  subsequently 
be  followed. 

Directions. — The  parotid  gland  should  now  be  removed  in  order  to 
expose  the  objects  beneath  it.  Its  removal  must  be  effected  with  great 
care,  so  as  to  leave,  as  far  as  possible,  the  vessels  and  nerves  which  lie 
beneath  it,  or  pass  through  its  substance.  This  will  be  best  done  by 
following  the  7th  nerve,  and  the  transverse  facial  and  maxillo-muscular 
vessels,  which  emerge  at  the  anterior  edge  of  the  gland,  and  the  jugular 
vein,  which  passes  through  its  substance.  In  removing  the  gland,  its 
vessels  and  nerves  must  be  cut.  Its  arteries  are  derived  from  the 
external  carotid  or  its  collateral  branches ;  its  veins  empty  themselves 


166 


THE    ANATOMY    OF    THE    HOR«E. 


iuto  the  jugular  or  auricular  veins ;  its  nerves  come  from  the  7th  cranial 
nerve. 

The  deep  ftice  of  the  parotid  gland  is  related  to  the  following 
structures : — The  mastoid  insertion  of  the  mastoido-humeralis,  the  ter- 
minal tendon  of  the  sterno-maxillaris,  the  stylo-maxillaris,  the  digas- 
tricus  (upper  belly),  the  occipito-stjloid,  the  stylo-hyoid,  the  submax- 
illary gland,  the  great  cornu  of  the  hyoid  bone,  the  guttural  pouch,  the 
external  carotid  artery  (with  its  terminal,  and  some  of  its  collateral, 
branches),  the  initial  part  of  the  jugular  vein  (formed  by  the  junction  of 
the  superficial  temporal  and  internal  maxillary  veins),  and  the  7th 
nerve. 

The  Tendons  of  the  Mastoido-Humeralis  and  Sterno-Maxillaris. 
These  muscles  are  described  at  pages  152  and  145  respectively.  The 
two  tendons  are  connected  by  a  fibrous  expansion,  which  is  included 
between  the  parotid  and  submaxillary  glands.  When  they  have  been 
examined,  the  fibrous  expansion  and  the  tendon  of  the  sterno-maxillaris 
may  be  removed. 

The  Stylo-Maxillaris.  This  muscle  is  not  distinct  from  the  digas- 
tricus  (upper  belly).  It  anses  in  common  with  that  muscle  from  the 
styloid  process  of  the  occipital  bone,  and  it  is  inserted  into  the  angle  of 
the  jaw. 

Action. — To  depress  the  lower  jaw,  and  assist  in  opening  the  mouth. 

The  DiGASTRicus  (Plate  31).  This  muscle  will  be  only  imperfectly 
exposed  at  present.  It  consists  of  an  upper  and  a  lower  muscular  belly, 
with  an  intermediate  tendon.  The  lower  belly  will  be  met  in  the  dis- 
section of  the  intermaxillary  space  ;  and  the  tendon,  in  the  dissection  for 
the  exposure  of  the  mouth  and  pharynx.  The  upper  belly  arises  from 
the  styloid  process  of  the  occipital  bone,  in  front  of  the  origin  of  the 
preceding  muscle,  with  which  it  is  confounded.  It  is  succeeded  b}-  the 
intermediate  tendon,  which  plays  through  a  perforation  in  the  tendon  of 
the  stylo-hyoid,  and  is  continued  by  the  lower  belly.  The  latter  becomes 
inserted  into  the  edge  of  the  inferior  maxilla,  behind  the  symphysis. 

Action. — To  depress  the  lower  jaw  and  open  the  mouth. 

The  Occipito-Styloid  (Plates  31  and  32).  This  muscle  arises  from 
the  front  of  the  styloid  process  of  the  occipital  bone  ;  and  it  is  inserted 
into  the  extremity  of  the  styloid  (great)  coi-nu  of  the  hyoid  bone,  behind 
its  point  of  articulation  with  the  skull.  The  deep  face  of  the  muscle  is 
lined  by  the  mucous  membrane  of  the  guttural  pouch ;  and  in  the 
operation  for  opening  the  pouch,  the  muscle  is  perforated. 

Action. — To  flex  the  temporo-hyoideal  joint,  and  carry  backwards  the 
hyoid  bone  and  the  parts  attached  to  it. 

The  Stylo-Hyoid  (Plate  31).  Only  the  origin  of  this  muscle,  from  the 
heel-like  part  of  the  great  cornu,  will  at  present  be  seen.  It  should  be 
identified,  and  preserved  for  examination  at  a  later  stage. 


DISSECTION    OF   THE    HEAD    AND    NECK.  167 

The  submaxillary  gland  is  to  be  preserved  without  disturbance  until 
it  can  be  exposed  in  its  entirety.  The  guttui-al  pouch  will  be  described 
to  more  advantage  at  a  later  stage. 

The  External  Carotid  Artery  (Plate  28).  Only  the  termination  of 
that  vessel  is  here  seen.  It  is  one  of  the  terminal  branches  of  the 
common  carotid,  which  divides  above  the  cricoid  cartilage,  under  cover 
of  the  submaxillary  gland.  The  first  part  of  the  artery — at  present  con- 
cealed— rests  on  the  guttural  pouch,  and  is  covered  by  the  stylo-max- 
illaris,  digastricus,  and  stylo-hyoid  muscles.  As  now  seen,  it  appears 
between  the  last  of  these  muscles  and  the  hinder  edge  of  the  great 
cornu ;  and  crossing  obliquely  upwards  and  forwards  on  the  surface  of 
that  bone,  it  terminates  by  dividing  into  the  superficial  temporal  and 
internal  maxillary  arteries.  The  vessel  detaches  three  collateral 
branches,  viz.,  the  submaxillary,  maxillo-muscular,  and  posterior  auricu- 
lar arteries,  of  which  the  first  is  concealed  by  the  digastricus  and  stylo- 
hyoid muscles. 

The  Maxillo-Muscular  Artery.  This  branch  is  given  off  at  the 
upper  edge  of  the  stylo-maxillaris  muscle,  immediately  after  the  parent 
trunk  emerges  between  the  stylo-hyoid  muscle  and  the  great  cornu.  It 
forms  with  the  continuation  of  the  main  trunk  a  very  obtuse  angle. 
Passing  forwards  and  downwards,  it  divides  into  an  outer  and  an  inner 
branch,  which  embrace  between  them  the  edge  of  the  vertical  ramus. 
The  outer  branch  appears  on  the  face  at  the  anterior  edge  of  the  parotid 
gland,  and  enters  the  masseter  muscle.  The  inner  branch  passes  to  the 
internal  pterygoid  muscle. 

The  Posterior  Auricular  Artery.  This  vessel  has  its  origin  a  little 
beyond  the  preceding,  but  from  the  opposite  side  of  the  carotid.  Its 
distribution  to  the  ear  is  given  at  page  163. 

The  Superficial  Temporal  Artery  (Plate  28).  Originating  by  the 
division  of  the  external  carotid  on  the  great  cornu,  this  artery,  after  a 
very  short  course  below  and  behind  the  condyle  of  the  lower  jaw, 
divides  into  the  transverse  facial  and  anterior  auricular  arteries. 

The  Transverse  Facial  Artery  turns  round  the  ramus  below  the 
condyle,  and  will  be  followed  in  the  dissection  of  the  face. 

Tne  Anterior  Auricular  Artery  ascends  to  the  front  of  the  ear,  on 
the  capsular  ligament  of  the  temporo-maxillary  joint.  Its  distribution 
is  given  at  page  162. 

The  Internal  Maxillary  Artery.  This,  much  the  larger  terminal 
branch  of  the  external  carotid,  passes  within  the  condyle  of  the  lower 
jaw,  where  it  will  subsequently  be  followed. 

Veins. — The  jugular  vein  is  formed  within  the  substance  of  the 
parotid  gland,  close  behind  the  articulation  of  the  jaw,  and  superficial 
to  the  termination  of  the  external  carotid,  a  few  lobules  of  the  gland 
separating  the  artery  and  vein.     The  vessels  which  imite  to  form  it  are 


168  THE  ANATOMY  OF  THE  HORSE. 

the  superficial  temporal  and  internal  maxUlary  veins.  It  passes  out 
throiigh  the  substance  of  the  parotid,  and  then  lies  in  a  groove  on 
its  surface,  where  it  receives  maxillo-musctdar  and  posterior  auricular 
branches. 

The  7th  Cranial  Nerve  (Portio  dura)  (Plate  28)  emerges  from 
the  aqueduct  of  Fallopius  by  the  stylo-mastoid  foramen  of  the  petroiis 
temporal  bone.  It  passes  dowTiwards  and  forwards  at  the  inner  face  of 
the  parotid  or  within  its  substance ;  and  txirning  round  the  inferior 
maxilla,  it  reaches  the  fece  with  the  transverse  facial  vessels.  In  this 
course  it  is  crossed  superficially  by  the  posterior  auricular  artery,  and 
passes  over  the  angle  of  division  of  the  superficial  temporal  artery.  As 
it  turns  roiind  the  inferior  maxilla,  it  is  joined  by  the  sensory  sub- 
zygomatic  branch  from  the  inferior  maxillary  division  of  the  5th  cranial 
nerve.  In  this  part  of  its  course  the  7th  nerve  detaches  the  following 
branches : — 

1.  The  Anterior,  Middle,  and  Posterior  Aiiricular  Nerves. — The  first  two 
are  given  off  at  the  st3do-mastoid  foramen,  the  last  is  detached  midway 
between  the  foramen  and  the  edge  of  the  ramus.  The  nerves  ascend  in 
or  beneath  the  parotid  gland,  and  their  distribution  is  given  at  page  163. 

2.  Nerves  to  the  occipito-styloid,  stylo-hyoid,  digastricus  (upper 
belly),  and  stylo-maxillaris  muscles.  These  are  given  off  at  the  stylo- 
mastoid foramen. 

3.  The  Cervical  Branch,  which  is  given  off  at  nearly  the  same  point  as 
the  anterior  auricular,  but  from  the  opposite  side  of  the  trunk.  It 
passes  through  the  parotid,  and  reaches  the  surface  of  the  gland  (see 
page  163). 

4.  Numerous  small  and  irregular  branches  to  the  parotid  gland  and 
guttural  pouch. 

The  Subzygomatic  Nerve  {Auriculo-temporal  of  man)  is  a  branch 
of  the  inferior  maxillary  division  of  the  5th  nerve,  given  off  at 
the  foramen  lacerum  basis  cranii.  It  descends  behind  the  capsular 
ligament  of  the  jaw ;  and  crossing  over  the  termination  of  the  super- 
ficial temporal  artery,  it  joins  the  7th  as  it  turns  round  the  ramus.  It 
sends  a  branch  to  accompany  the  transverse  facial  vessels  (Plate  29). 

THE    REGION    OF    THE    POLL. 

Directions. — It  will  be  convenient  at  this  stage  to  dissect  a  group  of 
muscles  (with  their  nerves  and  vessels)  placed  above  the  occipito-atlantal 
and  atlanto-axial  joints  (fig.  18,  page  156).  The  cervico-auricular  muscles 
of  one  side  having  been  cleared  away,  the  mastoid  tendon  of  the 
mastoido-humeralis  (page  152)  will  present  itself.  Beneath  that,  again, 
is  the  mastoid  tendon  common  to  the  splenius  and  trachelo-mastoideus 
(page  153);  and  still  deeper,  there  is  the  occipital  insei'tion  of  the 
complexus.     Each  of  these,  having  been  identified,  may  be  cut  away; 


DISSECTION    OF    THE    HEAD    AND    NECK.  169 

and  the  following  muscles  are  to  be  isolated,  the  nerves  and  vessels 
being  thereafter  dissected  on  the  other  side. 

The  Obliquus  Capitis  Inferior.  This  muscle,  the  most  powerful  of 
the  group,  covers  the  atlan to-axial  joint  on  each  side.  It  arises  from  the 
superior  spine  of  the  dentata;  and  it  is  inserted  into  the  wing  of  the 
atlas  on  its  upper  aspect. 

Action. — To  rotate  the  atlas  (and  head)  aroimd  the  odontoid  process 
of  the  axis. 

The  Obliquus  Capitis  Superior.  This  muscle  covers  the  occipito- 
atlantal  joint  on  each  side.  It  a7-ises  from  the  free  edge  of  the  wing  of 
the  atlas ;  and  it  becomes  inserted  into  the  mastoid  crest  and  styloid 
process  of  the  occipital  bone. 

Action. — To  extend  the  head  on  the  atlas. 

The  Rectus  Capitis  Posticus  Major.  This  muscle  is  composed  of 
two  parallel  portions  which,  although  not  distinctly  separated  from  one 
another,  were  described  as  distinct  muscles  by  Percivall.  It  arises  from 
the  spinous  process  of  the  axis.  Its  most  superficial  fibres  (complexus 
minor  of  Percivall)  join  the  occipital  insertion  of  the  complexus;  while 
its  deeper  portion  (rectus  capitis  posticzis  major  of  Percivall)  is  inserted 
into  the  back  of  the  occipital  bone,  beneath  the  insertion  of  the  com- 
plexus. 

Action.— -The  same  as  the  preceding  muscle. 

The  Rectus  Capitis  Posticus  Minor.  This  is  the  smallest  muscle  of 
the  group.  It  lies  beneath  the  preceding,  and  covers  the  occipito-atlantal 
joint.  It  arises  from  the  upper  aspect  of  the  ring  of  the  atlas;  and  it  is 
inserted  into  the  back  of  the  occipital  bone,  beneath  the  last-described 
muscle. 

Action. — The  same  as  the  two  preceding  muscles. 

The  1st  Cervical  Nerve  (Suboccipital  of  man)  issues  from  the  spinal 
canal  by  the  antero-internal  foramen  of  the  atlas.  At  its  point  of 
emergence  it  resolves  itself  into  superior  and  inferior  primary  branches. 
The  latter  is  immediately  directed  down  through  the  antero-external 
foramen,  and  will  be  followed  at  a  later  stage.  The  superior  primary 
branch  appears  between  the  obliquus  capitis  superior  and  the  rectus 
capitis  posticus  muscles,  where  it  gives  (1)  muscular  branches  to  these 
muscles,  and  (2)  auricular  branches  already  followed  to  the  skin  of  the 
ear. 

The  2nd  Cervical  Nerve  issues  by  the  foramen  at  the  anterior  edge  of 
the  arch  of  the  axis,  where  it  is  covered  by  the  obliquus  capitis  inferior.  It 
divides  into  superior  and  inferior  primary  branches,  the  latter  of  which 
has  already  been  referred  to  (page  144).  The  superior  branch  gives  twigs 
to  the  superior  and  inferior  oblique  muscles  of  the  head,  and  is 
continued  like  the  succeeding  members  of  the  cervical  series  (page  154). 

The  Occipital  Artery.     This  vessel  will  be  found  ascending  through 


170  THE  ANATOMY  OP  THE  HORSE. 

the  antero-cxtcrnal  foramen  of  the  atlas,  and  dividing  there  into  cerebro- 
spinal and  occipito-musndar  branches.  The  former  enters  the  spinal 
canal  by  the  antero-internal  foramen ;  the  latter  divides  for  the  supply 
of  the  muscles  and  other  stnxctiires  of  the  poll. 

The  Retrograde  or  Anastomotic  branch  of  the  occipital  artery  will  be 
found  issuing  with  a  backward  course  from  the  posterior  foramen  of  the 
atlas,  and  inosculating  with  the  termination  of  the  vertebral  artery. 

The  Mastoid  branch  of  the  occipital  artery  will  be  found  beneath 
the  obliquus  capitis  superior.  It  ascends  behind  the  styloid  process; 
and  crossing  over  the  mastoid  crest,  immediately  above  the  mastoid 
process,  it  passes  imder  the  edge  of  the  squamous  temporal  bone,  and 
enters  the  parieto-temporal  conduit,  in  which  it  anastomoses  with  the 
spheno-spinous  branch  of  the  internal  maxillary  artery. 

Veins. — Satellite  veins  accompany  these  arteries. 

THE    INTERMAXILLARY    SPACE. 

Directions. — Incise  the  skin  along  the  middle  line,  from  the  mental 
symphysis  upwards,  and  raise  it  on  each  side  as  far  as  the  edges  of  the 
rami. 

Cutaneous  Nerves.  The  skin  of  the  intermaxillary  space  is  supplied 
by  a  nerve  derived  from  the  2nd  cervical  nerve.  It  comes  from  the  first 
stellate  group  of  cutaneous  nerves  already  seen  on  the  surface  of  the 
mastoido-humeralis ;  and  crossing  obliquely  downwards  into  the  space 
(the  long  axis  of  the  head  is  supposed  to  be  vertical),  it  extends  to 
near  the  symphysis  of  the  lower  jaw. 

The  Panniculus.  This  is  here  extremely  thin.  It  hardly  forms  a 
continuous  layer,  but  consists  of  muscular  fasciculi  scattered  in  an  apon- 
eurosis. 

The  Submaxillary  Lymphatic  Glands  (Plate  27).  This  group  of 
glands  is  placed  on  the  inner  side  of  the  horizontal  ramus,  a  little  above 
the  point  where  its  edge  is  crossed  by  the  submaxillary  vessels  and 
Stenson's  duct.  It  rests  on  the  lower  belly  of  the  digastricus  muscle, 
being  related  inwardly  to  the  insertion  of  the  subscapulo-hyoid  mxiscle, 
and  outwardly  to  the  submaxillary  artery.  The  right  and  left  groups 
extend  towards  each  other,  and  nearly  meet  below  the  extremity  of 
the  glossal  (spur)  process  of  the  hyoid  bone.  These  glands  are  placed 
on  the  track  of  the  lymphatic  vessels  coming  from  the  mouth  and  nose ; 
and  in  morbid  states  of  these  parts,  such  as  glanders,  the  glands  become 
inflamed  and  enlarged  from  the  irritant  matters  conveyed  in  the  lymphatic 
vessels.  They  should  be  carefully  excised  to  expose  the  submaxillary 
artery  and  the  inferior  belly  of  the  digastricus  muscle. 

The  Submaxillary  or  Facial  Artery  (Plate  27)  appears  at  the 
upper  part  of  the  space,  descending  between  the  subscapulo-hyoid  and 
internal  pterygoid  muscles.       At    this  point  the  inferior  extremity  of 


DISSECTION    OF    THE    HEAD    AND    NECK.  171 

the  submaxillary  salivary  gland  lies  internal  to  it.  In  passing  obliquely 
backwards  and  downwards,  it  rests  on  the  internal  pterygoid  muscle,  and 
is  partly  covered  by  the  lymphatic  glands,  under  cover  of  which  it  de- 
taches its  submental  branch.  It  then  comes  into  company  with  the  vein 
of  the  same  name,  and  with  Stenson's  duct ;  and  the  three  vessels  turn 
round  the  edge  of  the  ramus  to  reach  the  face,  the  artery  being  below, 
the  vein  in  the  middle,  and  the  duct  superior.  Where  the  artery  turns 
round  the  ramus,  it  is  very  favourably  placed  for  taking  the  pulse,  since 
it  is  a  vessel  of  considerable  size,  is  in  an  easily  accessible  position,  rests 
directly  on  the  bone,  and  is  almost  subcutaneous,  only  the  thin  panni- 
culus  intervening  between  it  and  the  skin. 

The  Submental  Artery  crosses  downwards  beneath  the  inferior  belly  of 
the  digastricus,  then  along  the  surface  of  the  raylo-hyoideus,  which  it 
perforates  a  few  inches  above  the  symphysis.  It  will  be  followed  to  its 
termination  in  the  dissection  of  the  mouth. 

The  Submaxillary  Vein  (Plate  27)  is  in  contact  with  the  artery  where 
the  vessels  turn  round  the  ramus  ;  but  as  it  passes  backwards,  it  recedes 
slightly  from  the  artery,  and  follows  the  posterior  border  of  the  sub- 
maxillary gland.  It  leaves  the  space  above  the  angle  of  the  jaw,  and  is 
continued  along  the  lower  edge  of  the  parotid  to  join  the  jugular.  At  the 
lymphatic  glands  it  receives  the  suhmental  vein,  a  larger  vessel  than  the 
artery  of  the  same  name. 

Stenson's  Duct  (Plate  27),  after  crossing  the  sterno-maxillaris  tendon, 
passes  into  the  space,  at  the  posterior  edge  of  the  submaxillary  salivary 
gland.  It  passes  downwards  on  the  internal  pterygoid  muscle,  placing 
itself  in  contact  with  the  submaxillary  vein,  in  company  with  which  and 
the  artery  it  turns  round  the  bone  to  reach  the  face.  It  is  here  the  most 
superior  of  the  three  vessels ;  and  from  its  being  superficially  placed 
and  resting  on  the  bone,  it  is  liable  to  be  opened  when  a  blow  is  delivered 
over  this  region. 

The  Digastricus.  This  muscle  is  named  from  its  having  two  fleshy 
bellies,  with  an  intermediate  tendon.  The  upper  belly  arises  from  the 
styloid  process  of  the  occipital  bone,  being  confounded  with  the  stylo- 
maxillaris.  It  is  succeeded  by  the  intermediate  tendon,  which  plays 
through  the  tendon  of  the  styio-hyoid  muscle  ;  but  this  and  the  upper 
belly  are  at  present  concealed  within  the  jaw.  The  lower  belly  is  placed 
in  the  intermaxillary  space,  where  it  is  partly  covered  by  the  lymphatic 
glands,  and  is  related  anteriorly  to  the  mylo-hyoid  muscle.  It  is  inserted 
by  a  flat  fasciculated  tendon  into  the  edge  of  the  horizontal  ramus,  a  little 
distance  above  the  symphysis. 

Action. — To  depress  the  lower  jaw  and  open  the  mouth. 

The  MvLO-HYOiD  Muscles  stretch  across  the  intermaxillary  space,  and 
form  a  support  for  the  tongue.  Each  muscle  arises  from  a  line  on  the 
inner  surface  of  the  horizontal  ramus  behind  its  alveolar  border  ;  and  its 


172  THE  ANATOMY  OF  THE  HORSE. 

fibres  pass  transversely  inwards,  the  most  superior  getting  inserted  into 
the  body  and  glossal  process  of  the  hyoid  bone,  and  the  others  into  a 
median  fibrous  raphe  between  the  two  muscles.  The  muscle  is  to  be 
left  intact  at  present,  and  its  attachments  will  be  better  seen  in  the 
dissection  of  the  mouth. 

Action. — To  raise  the  body  of  the  tongue  towards  the  roof  of  the 
mouth,  and  thus  assist  in  mastication  and  deglutition. 

Nerve  to  the  mylo-hyoid  and  inferior  belly  of  the  digastric.  This 
is  a  branch  of  the  inferior  maxillary  division  of  the  5th  cranial  nerve. 
It  descends  between  the  internal  pterygoid  muscle  and  the  vertical  ramus ; 
and  passing  above  the  upper  edge  of  the  mylo-hyoid,  it  nins  do\rawards 
on  the  surface  of  that  muscle,  in  company  with  the  submental  artery. 
In  part  of  its  course  it  is  covered  by  the  lower  belly  of  the  digastricus, 
and  it  sends  to  that  muscle  a  distinct  branch,  which  enters  it  on  its 
outer  side. 

the  appendages  of  the  eye. 

These  are — the  eyelids,  the  membrana  nictitans,  the  caruncula 
lachrymalis,  the  conjunctival  membrane,  and  the  lachrymal  apparatus. 
The  lachrymal  gland — the  most  important  part  of  the  last  mentioned 
apparatus— will  be  dissected  with  the  interior  of  the  orbit;  but  the 
other  structures  enumerated  are  to  be  examined  now. 

The  Eyelids.  The  front  of  the  eye  is  protected  by  two  movable  cur- 
tains— the  upper  and  lower  eyelids ;  and  at  the  inner  side  of  the  eye  there 
is  placed  another  structure — the  membrana  nictitans,  which  plays  the 
part  of  a  third  eyelid.  The  upper  lid  is  larger  than  the  lower,  and  has 
a  greater  range  of  movement.  Each  ej'^elid  presents  two  surfaces,  two 
borders,  and  two  extremities.  The  outer  or  fecial  surface  is  formed  by 
a  continuation  of  the  skin,  and  is  covered  by  short  hairs.  Among  these 
there  occur  in  the  lower  lid  some  long  tactile  bristles.  The  inner  or  ocular 
surface  is  lined  by  the  conjunctival  membrane,  and  is  moulded  on  the 
front  of  the  eye.  If  the  upper  lid  be  everted  there  will  be  found  on  its 
inner  surface,  near  the  outer  angle,  a  number  of  minute  openings,  into 
which  bristles  should  be  passed.  These  are  the  openings  of  the 
excretory  ducts  of  the  lachrymal  gland.  On  the  same  surface,  but  near 
the  opposite  angle,  and  close  to  the  free  edge  of  each  lid,  there  is  a 
round  opening  of  larger  size,  but  still  minute.  These  are  the  puncta 
lachrymalia,  the  orifices  of  the  lachrymal  ducts,  by  which  the  lachrymal 
secretion  is  conveyed  away  from  the  eye.  The  free  borders  of  the  eyelids 
circumscribe  the  palpebral  fissure,  which  is  a  mere  line  when  the  eye  is 
closed,  but  is  ovoid  or  elliptical,  with  the  long  axis  directed  obliquely 
upwards  and  outwards,  when  the  e^^e  is  open.  The  free  edge  of  each  lid 
is  somewhat  stiff",  this  stiffiiess  being  due  to  a  slender  rod  of  cartilage 
which  extends  along  it.  The  meibomian  glands  are  lodged  in  depressions 
on  the  ocular  surface  of  this  cartilage,  and  may  be  seen  through  the 


DISSECTION    OP    THE    HEAD    AND    NECK.  173 

conjunctiva  as  close-set  yellow  lines  having  a  direction  at  right  angles  to 
the  edge  of  the  lid.  They  number  about  fifty  or  sixty  in  the  upper  lid, 
but  they  are  fewer  and  less  distinct  in  the  lower.  Each  gland  consists 
of  a  main  tube  with  lateral  sacculi  opening  into  it  on  each  side,  and  it 
discharges  its  secretion  by  a  dot-like  orifice  on  the  edge  of  the  eyelid. 
The  free  edge  of  each  lid  carries  a  fringe  of  stiff  hairs — the  eyelashes, 
which  tend  to  prevent  the  entrance  of  foreign  particles  into  the  eyes. 
The  attached  edge  of  each  lid  is  max-ked  on  the  ocular  side  by  the  angle 
of  reflection  of  the  conjunctiva  from  the  lid  to  the  eyeball,  but  on  the 
facial  side  the  eyelid  passes  into  the  surrounding  skin  without  any 
defined  line.  At  each  extremity  the  eyelids  join  to  form  a  commissure, 
or  canthus.  The  outer  or  temjwral  canthus  is  acute,  but  the  inner  or 
nasal  canthtis  is  rounded,  and  lodges  the  caruncula  lachrymalis. 

The  Caruncula  Lachrymalis  is  a  small,  rounded,  and,  generally, 
dark-pigmented  nodule  placed  within  the  nasal  canthus,  and  about 
equidistant  from  the  two  puncta  lachrymalia.  It  is  covered  by  con- 
junctiva, and  is  composed  of  connective-tissue  with  some  mucous  follicles 
and  the  bulbs  of  a  few  short  hairs,  which  project  from  it. 

The  Membrana  Nictitans.  This  body  is  placed  at  the  inner  canthus, 
where,  ordinarily,  it  projects  to  only  a  slight  extent,  but  it  is  capable  of 
being  thrust  more  than  half  way  across  the  front  of  the  eye.  It  has  for 
its  basis  a  thin  and  flexible  piece  of  elastic  cartilage,  which  anteriorly  is 
invested  by  conjunctiva.  Posteriorly  this  cartilage  passes  to  the  inner 
side  of  the  eyeball,  where  it  becomes  connected  with  the  cushion  of 
semifluid  fat  which  is  found  in  the  posterior  part  of  the  orbit.  The 
membrana  nictitans  has  no  muscle  to  move  it  directly;  but  when  the 
eyeball  is  retracted  within  the  orbit,  it  presses  on  the  semifluid  fat 
behind  it,  and  this,  tending  to  escape  at  the  side  of  the  eyeball,  pushes 
the  membrana  nictitans  before  it.  In  the  eye  of  a  subject  just  dead, 
this  mechanism  may  readily  be  demonstrated  by  pressing  the  eyeball 
backwards  into  the  orbit.     About  the  centre  of  the  outer  face  of  the 

cartilage,  there  will  be  found  a  cluster  of  reddish-yellow  granules the 

Harderian  gland.  The  gland  secretes  an  unctuous  material  which  is 
discharged  by  a  number  of  ducts  that  perforate  the  cartilage  and  open 
on  its  ocular  surface. 

The  Lachrymal  ApjMratus  comprises — the  lachrymal  gland  with  its 
excretory  ducts,  the  puncta  lachrymalia,  the  lachrymal  canals,  the 
lachrymal  sac,  and  the  lachrymal  duct. 

The  Lachrymal  Gland  is  placed  within  the  orbit,  beneath  the  supra- 
orbital process  of  the  frontal  bone.  The  gland  itself  will  be  dissected  at 
a  later  stage. 

The  excretory  ducts  of  the  gland  discharge  themselves  by  a  number 
of  minute  openings  on  the  inner  surface  of  the  upper  eyelid,  close  to  the 
temporal  canthus.     Sometimes  a  few  of  the  ducts  open  on  the  lower  lid 


17-i  THE  ANATOMY  OF  THE  HORSE. 

close  to  the  same  canthus.  The  watery  secretiou  which  issues  from 
them  is  carried  over  the  front  of  the  eyeball  by  the  movements  of  the 
eyelids,  and  at  the  nasal  canthus  it  is  drained  away  by  the  puncta 
lachrymalia. 

The  Puncta  Lachryjialia.  Each  punctum  is  placed  on  the  inner 
surface  of  the  lid  near  its  free  edge,  and  distant  about  i  of  an  inch  from 
the  caruucula.  The  lower  punctum  is  generally  larger  and  more  easily 
found  than  the  upper.  If  a  flexible  bristle  be  passed  into  each  punctum, 
it  may  be  directed  along  the  lachrymal  canal,  into  the  lachrymal  sac. 
Taking  the  bristle  as  a  guide,  each  canal  may  then  be  slit  open  with 
scalpel  or  scissors. 

The  Lachrymal  Canals  will  be  found  to  converge  towards  the  roof  of 
the  lachrymal  sac,  into  which  they  open  by  distinct  orifices.  The  upper 
canal  is  a  little  longer  than  the  lower. 

The  Lachrymal  Sac  is  a  small  reservoir  lodged  in  the  fossa  of  the 
same  name  on  the  orbital  surfoce  of  the  lachrymal  bone.  It  receives  the 
lachrymal  secretion  from  the  lachrymal  canals,  and  it  is  directly 
continued  as  the  lachrymal  duct. 

The  Lachrymal  Duct  {ductus  ad  luiswn)  conveys  the  lachrymal  secre- 
tion from  the  sac  to  the  lower  part  of  the  nasal  fossa.  In  the  first 
part  of  its  course  it  is  lodged  in  an  osseous  canal,  along  which  it  passes 
to  reach  the  middle  meatus  of  the  nose,  where  it  will  subsequently  be 
found.  The  lachrymal  canals,  sac,  and  duct  have  a  fibrous  wall  with  a 
mucous  lining,  the  epithelium  being  stratified  and  squamous  in  the  canals, 
but  ciliated  in  the  sac  and  greater  part  of  the  duct. 

The  Conjunctiva.  This  is  a  mucous  membrane,  consisting  of  a 
■palpebral  part  lining  the  inner  aspect  of  the  eyelids,  and  an  ocular 
portion  which  is  reflected  on  the  front  of  the  eyeball.  The  palpebral 
portion,  including  that  covering  the  caruncula  and  membrana  nictitans, 
consists  of  a  stratified  epithelium  and  a  papillated  layer  of  vascular 
subepithelial  connective-tissue  containing  small  mucous  glands.  The 
ocular  portion  where  it  covers  the  sclerotic  resembles  the  preceding  in 
structure,  but  is  thinner,  nonpapillated,  and  less  vascular;  wher^it 
passes  over  the  cornea,  it  consists  of  the  epithelium  only,  which  i^ 
generally  enumerated  as  one  of  the  constituent  layers  of  the  cornea 
itself,  being  tenned  its  antenor  e2ntheliu7)i.  Through  the  puncta 
lachrymalia,  the  conjunctival  epithelium  is  continuous  with  that  lining 
the  lachi-ymal  canals  ;  and  at  the  free  margin  of  the  lids  it  is  continuous 
with  the  epidermis. 

Structure  of  the  Eyelids. — Each  lid  is  composed  of  the  following 
parts : — a  layer  of  skin  outwardly,  the  palpebral  conjunctiva  inwardly, 
a  portion  of  the  orbicularis  palpebrarum  muscle,  the  palpebral  tendon, 
the  tarsal  cartilage,  vessels,  and  nerves. 

Besides  these,  there  is  found  in  the  upper  lid  the  tendon  of  a  special 


DISSECTION    OF    THE    HEAD    AND    NECK.  175 

muscle — the  levator  palpebi-pe  superioris.     Of  these,  the  skin  and  con- 
junctiva have  already  been  sufficiently  noticed. 

The  Orbicularis  Palpebrarum  (Fig.  19,  page  161).  This  muscle  will 
be  exposed  by  removing  the  layer  of  skin  from  the  eyelids,  and  from 
around  the  orbital  rim  for  the  breadth  of  an  inch.  The  fibres  of  the 
muscle  are  closely  adherent  to  the  skin  of  the  eyelids,  and  have  a 
circular  or  elliptical  disposition  around  the  palpebral  fissure.  Below  the 
nasal  canthus  a  number  of  the  fibres  are  inserted  by  a  slender  tendon 
into  the  lachr^-mal  tubei'cle  on  the  bone  of  the  same  name,  but  for  the 
most  part  the  fibres  are  without  bony  attachment.  Above  the  orbit  the 
muscle  is  confounded  witli  some  thin  fibres — the  corrugator  su2yercilii — 
which  wrinkle  the  overlying  skin. 

Action. — The  orbiculai-is  muscle  closes  the  eye  by  approximating  the 
free  edge  of  the  eyelids,  and  in  this  action  the  upper  lid  has  a  much 
wider  range  of  movement  than  the  lower. 

The  Palpebral  Tendon.  This  is  a  fibrous  layer  which  w'ill  be 
exposed  by  removing  the  orbicularis  muscle.  At  the  free  edge  of  the 
eyelid  it  is  margined  by  the  tarsal  cartilage,  while  by  its  opposite  border 
it  is  fixed  to  the  rim  of  the  orbit. 

The  Tarsal  Cartilage.  This  is  a  slender  rod  of  elastic  cartilage 
imbedded  in  the  free  edge  of  each  eyelid.  The  meibomian  glands  lie  in 
grooves  on  its  ocular  surface.  It  prevents  the  margins  of  the  lids  from 
being  drawn,  or  puckered,  when  the  ox'bicular  muscle  contracts. 

The  Levator  Palpebr.e  Superioris.  This  muscle  is  found  in  the 
upper  eyelid  only.  The  fleshy  portion  of  the  muscle  is  a  slender  flat 
fasciculus  which  takes  origin  at  the  back  of  the  orbit  (Fig.  22,  page  209) 
above  the  optic  foramen,  where  it  will  subsequently  be  exposed  along 
with  the  other  contents  of  the  cavity.  Passing  along  the  eyeball  and 
beneath  the  lachrymal  gland,  it  is  succeeded  by  a  thin  flat  tendon  which 
in  the  lid  is  placed  beneath  the  palpebral  tendon,  and  joins  the  tarsal 
cartilage. 

Action. — To  open  the  eye  by  raising  the  upper  lid.  In  this  move- 
V-  rri^it  the  muscle  plays  over  the  eyeball  like  a  rope  over  a  pulley.  The 
lower  lid,  it  is  to  be  observed,  has  no  analogous  muscle,  because,  in  the 
first  place,  it  is  raised  but  slightly  under  the  action  of  the  orbicularis, 
and,  secondly,  because  its  own  weight  and  elasticity  are  sufficient  to 
depress  it. 

Vessels.  The  arteries  of  the  eyelids  are  derived  from  the  supra- 
orbital and  lachrymal  arteries,  and  from  the  orbital  branch  of  the 
superior  dental  artery. 

The  Supra-Orbital  Artery  will  be  found  emerging  from  the  orbit 
by  the  supra-orbital  foramen.  It  is  a  branch  of  the  ophthalmic 
artery. 

The  Lachrymal  Artery   is  also  a  branch   of  the  ophthalmic,  and  is 


176 


THE  ANATOMY  OK  THE  HOHSE. 


distributed  mainly  to  the  gland  of  the  same  name,  but  it  sends  some 
twigs  to  the  upper  eyelid. 

The  Orbital  Branch  of  the  Sujjerior  Dental  Artery  creeps  over  the 
lower  part  of  the  rim  of  the  orbit,  and  descends  to  anastomose  with  the 
submaxillary  artery.     It  gives  some  twigs  to  the  lower  eyelid. 

Nerves.  The  sensory  nerves  of  the  eyelids  are  derived  from  the 
supra-orbital  and  palpebro-nasal  branches  of  the  ophthalmic  division  of 
the  5th  cranial  nerve,  and  from  the  orbital  branch  of  the  superior 
maxillary  division  of  the  5th.  The  supra-orbital  nerve  emerges  by  the 
foramen  of  the  same  name,  and  is  distributed  to  the  upper  eyelid,  and  to 
skin  around  its  point  of  exit.  The  palpebro-nasal  nen-c  sends  a  branch 
to  supply  the  lower  lid  and  the  strxictures  at  the  inner  canthus. 
The  orbital  branch  of  the  superior  maxillary  division  of  the  5th  nerve 
is  distributed  in  the  neighbourhood  of  the  outer  canthus.  The  motor 
nerve  to  the  orbicularis  comes  from  the  7th,  which  crosses  internal  to 
the  nasal  canthus.  The  motor  nerve  to  the  levator  palpcbrse  comes 
ft'om  the  3rd,  but  cannot  be  reached  at  present. 


THE    FACE. 

The  Nostrils,  or  the  Inferior  or  Anterior  Nares.     Some  points  in 
connection  witli   these  may  conveniently  be  noticed  before  the  student 

proceeds    to     dissect    the 
face.       The  nostril  is  the 

entrance     to      the     nasal 

chamber  or  fossa — the  first 
segment  of  the  respiratoiy 
passages.  It  is  a  large, 
somewhat  oval  opening 
bounded  laterally  by  the 
aloe,  or  mngs,  of  the  nostril. 
The  alsD  meet  above  and 
below%  forming  the  com- 
missures, the  lower  of 
which  is  wide  and  rounded, 
while  the  upper  is  acute. 
The  outer  ala  is  concave  in 
the  whole  of  its  extent; 
but  the  inner  ala,  while 
concave  below,  forms  a 
convex  projection  close  to 
the  upper  commissure.  If 
now  the  inner  wing  be  manipulated,  it  will  be  felt  to  contain  a  piece 
of  cartilage  which  begins  above  in  the  convex  projection  close  to  the 
upper  commissure,  extends  downwards  and  round  the  inferior  commis- 


FiG.  20. 

1,  1.  Coiuni.a-like  c.irtilages  of  the  m 


Septal  oaitilage. 


DISSECTION    OF    THE    HEAD    AND    NECK.  177 

sure,  and  terminates  in  the  lower  part  of  the  outer  ala.  This  alar  car- 
tilage, when  dissected  out,  displays  a  close  resemblance  to  a  comma,  the 
broad  part  being  placed  in  the  upper  part  of  the  inner  wing,  and  the 
point  in  the  lower  part  of  the  outer  wing.  The  cartilages  are  movably 
attached  by  fibrous  tissue  to  the  lower  extremity  of  the  septal  cartilage 
of  the  nose,  and  they  give  the  necessary  firmness  to  the  edges  of  the 
nostrils,  preventing  these  from  falling  together  in  the  act  of  inspiration. 
The  nostrils  are  covered  outwardly  by  skin  continuous  with  that  of  the 
face.  This  skin,  which  is  thin  and  adherent  to  the  subjacent  textures,  is 
carried  roimd  the  margins  of  the  alae,  and  for  a  short  distance  into  the 
nasal  chamber.  If  the  finger  be  introduced  below  the  upper  commis- 
sure, it  will  be  felt  to  enter  a  peculiar  diverticuhun  termed  the  false 
nostril.  This  blind  pouch  extends  upwards  for  about  four  inches,  and 
it  is  lined  by  a  continuation  of  the  skin.  If  the  wings  of  the  nostril  be 
separated  as  widely  as  possible,  the  opening  of  the  lachrymal  duct  will 
be  seen  a  few  inches  within  the  lower  commissure.  The  orifice  is  circu- 
lar, appearing  as  if  a  small  piece  of  skin  had  been  punched  out.  Just 
beyond  the  orifice  the  skin  is  continued  by  the  mucous  membrane  of 
the  nasal  chamber,  which  is  distinguished  from  the  common  integument 
by  being  non-pigmented,  vascular-looking,  and  destitute  of  hairs. 

Directions. — The  skin  on  the  face  should  be  entirely  removed.  Over 
the  lips  and  false  nostril  care  is  necessary  to  avoid  going  deeper  than  the 
skin.  One  side  should  first  be  used  for  the  muscles,  and  then  the  vessels 
and  nerves  should  be  dissected  on  the  opposite  side. 

The  muscles  now  to  be  dissected  include  a  series  of  thin  muscles 
which  terminate  in  and  move  the  lips  and  nostrils  ;  the  buccinator, 
which  forms  the  basis  of  the  cheek  ;  and  the  masseter,  a  powerful 
muscle  of  mastication,  covering  the  vertical  ramus  of  the  lower  jaw. 
A  reference  to  Plate  29  will  enable  the  dissector  to  identify  these 
muscles. 

The  Panniculus  Carnosus  does  not  form  a  continuous  muscle  on 
the  face.  It  consists  of  disconnected  bundles  developed  in  the  sub- 
cutaneous fascia.  It  is  best  developed  over  the  masseter  muscle,  where 
it  is  continuous  over  the  edge  of  the  inferior  maxilla  with  the  same 
structure  in  the  parotideal  and  intermaxillary  regions.  On  the  cheek  a 
few  bundles  reach  the  angle  of  the  mouth,  in  whose  retraction  they 
assist. 

Below  the  rim  of  the  orbit  there  is  found  a  thin  layer  of  pale  muscular 
fibres  and  intermediate  fascia,  which  may  be  viewed  as  a  part  of  the 
facial  panniculus.  It  wrinkles  the  overlying  skin.  This  is  the  lachry- 
mal muscle  of  French  authors,  and  the  inferior  palpebral  muscle  of  Leyh. 

The  Levator  Labii  Superioris  Al^eque  Nasi  (Plate  29).  This 
muscle  has  a  narrow,  thin,  and  aponeurotic  o^rigin  from  the  frontal  and 
nasal  bones,  below  and  internal  to  the  orbit.     It  passes  over  the  levator 

N 


178  THE  AXATOMY  OF  THE  HORSE. 

labii  superioris  proprius  ;  and  widening  as  it  descends,  it  divides  into 
an  anterior  and  a  posterior  branch,  between  which  the  lateral  dilator  of 
the  nostril  emerges.  The  anterior  is  the  larger  branch,  and  it  passes 
beneath  the  last-mentioned  muscle  to  end.  in  the  outer  wing  of  the 
nostril  and  the  adjacent  part  of  the  upper  lip.  The  posterior  branch  is 
inserted  into  tlie  angle  of  the  moiith. 

Action. — To  dilate  the  nostril  and  elevate  tlie  upper  lip  by  its 
anterior  branch ;  to  raise  the  angle  of  the  mouth  by  its  posterior 
branch. 

The  Levator  Labii  Superioris  Proprius  (Plates  29  and  30,  and  fig. 
19).  By  its  superior  extremity,  which  is  rounded  and  fleshy,  this 
muscle  takes  origin  below  the  orbit,  from  the  malar  and  superior  maxil- 
lary bones.  Becoming  narrower  and  thicker,  it  passes  downwards  and 
forwards  beneath  the  last-described  miiscle,  whose  direction  it  crosses. 
Appearing  in  front  of  this  muscle,  it  lies  on  the  false  nostril ;  and  here  its 
muscular  portion  is  succeeded  by  a  narrow  tendon,  which  passes  inwards 
above  the  superior  commissure  of  the  nostril  to  gain  the  iipper  lip.  In 
the  lip  it  passes  over  the  transverse  dilator  of  the  nose  ;  and  approach- 
ing the  muscle  of  the  opposite  side,  it  becomes  united  to  it  across  the 
middle  line  by  an  intermediate  fascia,  and  is  inserted  into  the  texture  of 
the  upper  lip. 

Action. — Acting  together,  the  right  and  left  muscles  elevate  the 
upper  lip  vertically.  When  the  muscle  of  either  side  acts  singly,  the 
lip  is  raised  and  inclined  towards  that  side. 

The  Dilatator  Naris  Lateralis.  (Doubtfully  the  homologue  of 
the  caninus  of  man)  (Plate  29).  This  muscle  is  triangular  in  form. 
It  is  narrow  and  pointed  at  its  upper  extremity,  where  it  arises  from 
the  superior  maxilla.  It  passes  between  the  two  branches  of  the  levator 
labii  superioris  alseque  nasi ;  and  Avidening  as  it  descends,  it  passes  over 
the  anterior  branch  of  that  muscle,  and  becomes  inserted  into  the  outer 
wing  of  the  nostril.  Its  most  posterior  fibres  blend  with  the  orbicularis 
oris. 

Action. — To  dilate  the  nostril  by  pulling  the  external  wing  outwards. 

The  Dilatator  Naris  Traksversalis  (Fig.  19).  This  is  a  four-sided, 
fleshy  miiscle,  whose  fibres  pass  transversely  across  the  middle  line,  and 
are  inserted  at  either  extremity  into  the  broad  part  of  the  comma-like 
cartilage  of  the  nostril.  It  is  partly  covered  by  the  termination  of  the 
levator  labii  superioris  proprius. 

Action. — To  dilate  the  nostrils  by  approximating  the  internal  wings  of 
opposite  sides. 

The  Dilatator  Naris  Superior.  This  muscle  comprises  a  few  pale 
fasciculi  which  arise  from  the  projecting  edge  of  tlie  septum  nasi  at  the 
side  of  the  nasal  peak,  and  terminate  in  the  wall  of  the  false  nostril,  and 
on  the  extremity  of  the  ethmoidal  (anterior)  turbinated  bone. 


DISSECTION    OF    THE    HEAD    AND    NECK.  179 

Action. — To  dilate  the  fixlse  nostril. 

The  Dilatator  Naris  Inferior.  The  fibres  of  this  muscle  caise  from 
the  free  edge  of  the  premaxillary  and  superior  maxillary  bones ;  and 
they  terminate  in  the  wall  of  the  false  nostril,  and  on  the  cartilaginous 
prolongation  of  the  maxillary'  (posterior)  turbinated  bone. 

Action. — The  same  as  the  preceding  muscle. 

The  Zygomaticus  (Plate  29)  is  a  riband-shaped  muscle,  often 
extremely  thin.  It  extends  vertically  between  its  point  of  origin  from 
the  surface  of  the  masseter,  behind  the  lower  portion  of  the  zygomatic 
ridge,  and  its  insertion  into  the  buccinator  above  the  angle  of  the  mouth. 

Action. — To  raise  the  angle  of  the  mouth. 

The  Buccinator  (Plate  29).  This  muscle  forms  the  main  mass  of 
the  cheek.  It  is  not  wholly  exposed  at  present,  its  upper  extremity 
being  covered  by  the  masseter.  It  comprises  a  superficial  and  a  deep 
portion,  the  former  being  separately  described  by  Percivall  as  the 
caninus.  The  supei-ficial  portion  shows  a  longitudinal  rcqihe  from  wdiich 
the  muscular  fibres  pass  forwards  and  backwards  in  a  penniform  manner. 
In  front  these  fibres  become  attached  to  the  superior  maxilla,  from  the 
alveolus  for  the  canine  tooth  to  that  for  the  first  molar  inclusive ;  and 
behind,  to  the  inferior  maxilla  opposite  the  interdental  space  between 
the  canine  and  the  first  molar.  The  deep  portion  of  the  muscle  is 
longer  but  narrower  than  the  preceding.  It  arises  from  the  superior 
maxilla  in  front  of  the  three  upper  molar  alveoli ;  from  the  scabrous 
imprint  on  the  same  bone  above  the  last  alveolus ;  and  from  the  edge 
of  the  inferior  maxilla  above  the  last  alveolus.  Inferiorly  the  fibres  of 
the  muscle  blend  with  the  orbicularis  oris  at  the  angle  of  the  mouth. 
As  will  be  seen  at  a  later  stage,  after  the  removal  of  the  masseter,  the 
superior  buccal  gland  lies  on  the  muscle  at  its  anterior  edge,  while  the 
inferior  buccal  gland  lies  under  its  posterior  edge,  and  the  mucous 
membrane  of  the  mouth  lines  its  inner  surface. 

Action. — When  unopposed  by  the  orbicularis  oris,  the  muscle  will 
retract  (elevate)  the  angle  of  the  mouth ;  but  otherwise,  as  in  mastica- 
tion, it  compresses  the  cheek  against  the  teeth  and  their  alveoli,  and 
tends  to  keep  the  food  between  the  xipper  and  the  lower  molars.  In 
man,  when  the  cheeks  are  distended,  as  in  blowing  a  wind  instrument, 
the  muscle  compresses  the  volume  of  air  and  propels  it  as  a  stream 
from  the  mouth  :  hence  the  name,  from  the  L.  huccina,  a  trumpet. 

The  Depressor  Labii  Infbrioris  (Plate  29).  This  muscle  is  placed 
along  the  hinder  edge  of  the  buccinator,  with  which  it  is  confounded  at 
its  origin  from  the  alveolar  edge  of  the  inferior  maxilla  above  the  last 
molar  tooth.     It  terminates  in  the  texture  of  the  lower  lip. 

Action. — To  depress  (retract)  the  lower  lip. 

The  Orbicularis  Oris  (Plate  29).  This  muscle  surrounds  the 
.aperture  of  the  mouth  like  a  sphincter.     It  cannot  be   described  as 


180  THE  ANATOMY  OF  THE  HORSE. 

having  either  origin  or  insertion,  its  fibres  forming  a  continuous  ellipse. 
In  both  lips  the  fibres  are  intimately  adherent  to  the  skin,  and  they  are 
pai'tially  blended  with  the  labial  insertions  of  some  of  the  muscles 
already  dissected. 

Action. — To  approximate  the  lips,  as  in  the  simple  act  of  closing  the 
mouth  or  in  prehension. 

Directions. — Evert  the  iipper  lip,  and  wipe  its  exposed  mucous  lining 
clean.  Observe  that  it  is  studded  with  numerous  short,  tubercle-like 
papillte.  Each  of  these  is  perforated  by  the  duct  of  a  labial  mucous 
gland  lying  beneath  the  mucous  membrane.  Now  dissect  away  the 
mucous  meml)rane  so  as  to  expose  these  glands.  At  the  same  time, 
there  will  be  brought  into  view  the  following  muscle  : — 

The  Depressor  Labii  Superioris.  Under  this  name,  Percivall 
describes  a  biuidle  of  muscular  fibres  that  in  the  human  subject  is 
reckoned  a  part  of  the  buccinator.  On  each  side  the  muscle  an.^es  from 
the  premaxillary  bone  above  the  corner  incisor  and  the  interdental 
space  as  far  as  the  canine  tooth ;  and,  on  the  other  hand,  its  fibres 
termiruite  in  the  upper  lip,  blending  with  the  orbicularis.  Branches  of 
the  infra-orbital  and  7th  nerves  enter  the  lip  between  the  outer  edge 
of  this  muscle  and  the  lowest  fibres  of  the  buccinator. 

Action. — To  assist  the  orbicularis  by  depressing  the  upper  lip. 

The  Palato-Labial  Artery.  While  the  upper  lip  is  kept  everted, 
dissect  backwards  on  the  middle  line  until  this  artery  is  found  coming 
forwards  from  the  roof  of  the  mouth  by  the  incisor  foramen.  At  its 
point  of  exit  it  bifm-cates,  its  branches  passing  right  and  left  to  anasto- 
mose with  the  superior  labial  artery. 

Directions. — Evert  the  lower  lip,  and  wipe  its  mucous  surface  clean. 
Notice  that  it  is  smooth,  with  few  or  none  of  the  tubercle-like  papillce 
found  on  the  upper  lip.  Dissect  away  the  mucous  membrane,  which  is 
intimately  adherent  to  the  orbicularis  muscle.  There  arc  few  or  no 
labial  mucous  glands  in  the  lower  lip. 

The  Levator  Menti.  This  is  the  name  given  by  Percivall  to  a 
muscle  of  the  lower  lip  resembling  the  deju-essor  already  dissected  in  the 
upper.  Its  fibres  arise  on  each  side  from  the  inferior  maxilla  beneath 
the  intermediate  and  corner  incisors,  and  from  the  interdental  space  as 
far  as  the  canine  tooth.  It  runs  downwards  and  backwards  to  terminate 
in  the  so-called  prominence  of  the  chin,  being  there  intermixed  with 
fibrous  tissue,  and  confounded  in  front  with  the  orbicularis  oris.  The 
inferior  labial  artery  entei-s  the  lip  between  the  outer  edge  of  the  muscle 
and  the  lower  fibres  of  the  buccinator. 

Action. — To  elevate  the  lower  lip. 

The  Masseter  (Plate  29).  This  muscle  covers  the  vertical  ramus  of 
the  lower  jaw^  It  has  a  flattened,  semicircular  form,  and  is  thick  and 
powerful.     In  its  anterior  half  the  surface  of  the  muscle  is  glistening 


DISSECTION    OP    THE    HEAD    AND    NECK.  181 

and  tendinous,  but  posteriorly  it  is  fleshy.  Other  strong  tendinous 
layers  are  included  within  the  substance  of  the  muscle,  and  give 
attachment  to  many  of  its  fasciculi.  The  muscle  arises  from  the 
zygomatic  ridge  of  the  malar  and  superior  maxillary  bones,  and  it 
becomes  inserted  into  the  outer  sxirfiice  of  the  vertical  rami;s. 

Action. — To  elevate  the  lower  jaw,  and  aid  in  mastication  by  bringing 
tlie  lower  teeth  forcibly  into  contact  with  the  iipper. 

The  Submaxillary  or  Facial  Artery  has  already  been  dissected  in 
the  intermaxillary  space.  It  reaches  the  face  by  turning  round  the 
edge  of  the  inferior  maxilla,  in  company  with  the  vein  of  the  same 
name  and  Stenson's  duct  (Plate  29).  Here  the  artery  is  the  most 
inferior  of  the  three  vessels,  and  the  dvict  is  the  most  superioi'.  The 
three  vessels  cross  the  inferior  maxilla  at  the  lower  edge  of  the  masseter 
muscle,  and  preserve  the  before-mentioned  relationship  until  they  pass 
on  to  the  buccinator.  Here  the  vessels  cross  over  the  duct,  being 
themselves  crossed  superficially  by  the  facial  nerves;  and  passing 
beneath  the  zygomaticus,  below  the  zygomatic  spine,  they  reach  the 
surface  of  the  superior  maxilla,  on  which  the  artery  divides  into  an 
ancjular  and  a  nasal  branch.  The  angular  branch  detaches  a  twig  to 
anastomose  with  the  orbital  branch  of  the  superior  dental  artery,  and  is 
then  distributed  to  the  muscles  and  skin  beneath  the  orbit.  The  nasal 
branch  reaches  the  false  nostril  by  passing  beneath  the  levator  labii 
superioris  alseque  nasi.  The  collateral  branches  which  the  submaxillary 
gives  off  in  this  part  of  its  course  are  the  superior  and  inferior  labial 
arteries ;  and  unnamed  cutaneous  or  muscular  branches,  of  which 
those  that  pass  to  the  masseter  muscle  are  the  largest. 

The  Inferior  Labial  Artery  is  the  largest  branch.  It  is  given  off  on 
the  surface  of  the  inferior  maxilla.  It  passes  beneath  the  depressor 
labii  inferioris,  and  descends  to  the  lower  lip,  at  the  hinder  edge  of  the 
buccinator.  At  the  mental  foramen  it  anastomoses  with  the  mental 
branch  of  the  inferior  dental  artery,  and  in  the  lip  it  anastomoses  on 
the  mesial  plane  with  the  opposite  vessel. 

The  Superior  Labial  Artery  is  detached  opposite  the  zygomatic  ridge. 
It  passes  beneath  the  levator  labii  superioris  alfcque  nasi  and  the  lateral 
dilator  of  the  nostril,  and  reaches  the  upper  lip,  in  which  it  anastomoses 
with  the  palato-labial  artery. 

The  Submaxillary  Vein  (Plate  29)  runs  in  close  company  with  the 
artery.  Where  the  vessels  appear  on  the  face,  and  throughout  the 
greater  part  of  their  course,  the  vein  is  above  the  artery.  It  is  formed 
by  an  angular  and  a  nasal  branch,  the  first  of  which  begins  in  the  lower 
eyelid,  while  the  other  originates  beneath  the  skin  of  the  false  nostril, 
and  passes  over  the  levator  labii  superioris  alreque  nasi  to  join  the  first. 
It  receives  branches  corresponding  to  those  of  the  artery,  and,  in  addi- 
tion, the  alveolar  and  buccal  veins  join  it  at  the  edge  of  the  masseter. 


182  THE  AXATOMY  OF  THE  HORSE. 

In  thin-skinned,  fine-bred  animals  the  course  of  the  vein  and  of  its 
angular  and  nasal  branches  is  conspicuous  during  life. 

Stexson's  Duct  (Plate  29),  or  the  parotid  duct,  crosses  the  face, 
being  at  first  between  the  edge  of  the  masseter  and  the  vein.  It  then 
passes  forwards  and  upwards  beneath  the  artery  and  vein,  on  the  surface 
of  the  buccinator  :  and  finally  it  perforates  that  muscle  under  cover  of 
the  zygomaticus,  and  opens  on  the  inner  surface  of  the  cheek,  opposite 
the  third  upper  molar  tooth. 

The  Transverse  Facial  Artery  (Plate  29)  is  a  branch  of  the 
superficial  temporal.  Given  off  beneath  the  surface  of  the  parotid 
gland,  it  turns  round  the  edge  of  the  ramus  beneath  the  condyle,  and 
at  the  anterior  border  of  the  gland  it  gives  off  a  large  masseteric  branch. 
Having  gained  the  face,  it  descends  for  a  few  inches  on  the  surface 
of  the  masseter  muscle,  immediately  below  the  zygomatic  arch ;  and  then, 
about  midway  between  the  temporo-maxillary  joint  and  the  orbit,  it 
penetrates  the  masseter,  and  is  distributed  to  it.  Where  the  vessel  is 
on  the  surface  of  the  muscle,  it  is  covered  only  by  the  thin  facial 
panniculus  and  the  skin,  and  is  conveniently  placed  for  the  taking  of 
the  pulse.  Its  masseteric  branch  at  once  plunges  into  the  sxibstance  of 
the  massctei',  where  it  anastomoses  with  the  maxillo-muscular  artery, 
and  with  the  posterior  deep  temporal  by  a  small  branch  which  traverses 
the  corono-condyloid  notch. 

The  Maxillo-Muscular  Artery  (Plate  29)  is  a  collateral  branch  of 
the  external  carotid.  Beneath  the  parotid  gland  it  bifurcates  to  form 
a  'pterygoid  and  a  masseteric  hrancli.  The  former  passes  Avithin  the 
ramus  to  reach  the  internal  pterygoid  muscle.  The  latter  emerges 
between  the  ramus  and  the  parotid  gland,  above  the  insertion  of  tlie 
stylo-maxillaris,  and  penetrates  the  masseter. 

Veins. — The  transverse  facial  and  maxillo-muscnlar  veins  accom- 
pany the  arteries  of  the  same  name.  The  former  joins  the  anterior 
auricular  to  form  the  superficial  temporal  vein,  the  latter  empties  itself 
into  the  jugular. 

The  7th  Nerve  on  the  face  (Plate  29).  This  nerve  appeai-s  on  the 
face  a  little  below  the  articulation  of  the  jaw,  whei-e  it  emerges  from 
beneath  the  parotid  gland.  Before  its  emergence  it  is  joined  by  the 
subzygomatic  branch  of  the  inferior  maxillary  division  of  the  5th.  The 
nerve  divides  into  a  variable  number  of  branches,  Avhich  anastomose  on 
the  sui-fiice  of  the  masseter  and  form  a  plexus,  termed  in  man  the  ^)c.s 
anserinvs  (from  its  resemblance  to  the  foot  of  a  goose).  In  this  plexus 
it  is  not  possible  to  distinguish,  among  the  motor  fibres  proper  to  the 
nerve  itself,  those  sensory  fibres  derived  from  the  subzygomatic  nei-ve. 
This  plexus  is  covered  by  the  skin  and  the  thin  facial  panniculus,  both 
of  which  receive  branches  from  it.  Below  the  inferior  edge  of  the 
masseter,  branches  of  the  plexus  are  continued  over  the  submaxillary 


DISSECTION    OF    THE    HEAD    AND    NECK  183 

vessels  to  supply  the  muscles  of  the  cheek,  lips,  and  nostrils,  as  well  as 
the  panniculus  and  skin.  The  largest  of  these  branches  is  the  most 
anterior,  and  it  passes  beneath  the  zygomaticus  muscle  in  company  with 
the  superior  labial  vessels.  It  communicates  with  the  infra-orbital 
nerves  emerging  from  the  infra-orbital  foramen,  and  passes  with  them 
to  the  upper  lip.  Another  branch  of  considerable  size  passes  beneath 
the  retractor  of  the  lower  lip,  and  runs  in  company  with  the  inferior 
labial  artery.  It  reaches  the  lower  lip,  in  which  it  is  distributed  along 
with  the  mental  nerves. 

The  SuBZYGOMATic  Branch  of  the  5th  nerve  (Plate  31).  The  major 
portion  of  this  nerve  joins  the  7th,  and  is  distributed  with  it  on  the 
face  ;  but  before  joining  with  that  nerve,  it  detaches  a  branch  which 
passes  in  company  with  the  transverse  facial  vessels,  and  is  traceable  as 
far  as  the  orbit. 

The  Infra-orbital  Nerve  (Plate  30).  This,  which  is  derived  from 
the  superior  dental  branch  of  the  sviperior  maxiUary  division  of  the 
5th,  and  is  therefore  sensory,  emerges  from  the  infra-oi'bital  foramen, 
under  cover  of  the  levator  labii  superioris  proprius  muscle.  The 
divisions  of  the  nerve  descend  beneath  the  levator  labii  superioris 
alseque  nasi  and  the  lateral  dilator  of  the  nostril,  where  they  com- 
municate with  a  branch  of  the  7th  nerve,  and  are  distributed  in  the 
nostril  and  iipper  lip. 

Directions. — The  masseter  muscle  on  one  side  is  now  to  be  removed  ; 
and  in  doing  this,  the  dissector  should  find  the  branch  from  the  inferior 
maxillary  division  of  the  5th  nerve  which  enters  the  muscle  through 
the  corono-condyloid  notch.  The  masseter  having  been  removed,  the 
buccinator  muscle  will  now  be  fully  exposed  (see  page  179),  and  the 
dissector  is  to  examine  the  alveolar  vein,  the  buccal  glands,  the  buccal 
nerve  and  vessels,  and  the  interior  of  the  cheek. 

The  Alveolar  Vein.  This  vessel  rests  on  the  superior  maxilla,  along 
the  alveoli  for  the  molar  teeth.  Inferiorly  it  joins  the  submaxillary 
vein,  and  is  here  comparatively  small.  It  speedily  becomes  of  large 
size,  and  it  is  continued  round  the  upper  extremity  of  the  superior 
maxilla  to  reach  the  orbit,  where  it  will  afterwards  be  followed. 

The  Superior  Buccal  Gland.  This  consists  of  a  string  of  lobules 
resting  on  the  anterior  edge  of  the  buccinator.  The  string  is  thickest 
above,  and  thinnest  below,  where  its  lobules  are  placed  at  intervals. 
The  ducts  of  the  lobules  perforate  the  buccinator,  and  will  afterwards 
be  seen  opening  on  the  cheek. 

The  Buccal  Artery  will  be  found  crossing  the  upper  end  of  the 
superior  buccal  gland.  It  is  distributed  to  the  buccinator  muscle  and 
the  other  textures  of  the  cheek.  Its  origin  from  the  internal  maxillary 
artery,  at  the  floor  of  the  orbit,  will  subsequently  be  dissected. 

The  Buccal  Vein  begins  at  the  inferior  edge  of  the  masseter,  where 


184  THE  ANATOMY  OP  THE  HORSE. 

it  joins  the  submaxillary  vein.     Above  the  cheek  it  is  continned  as  the 
internal  maxillary  vein. 

Directions. — The  buccinator  muscle  should  be  cut  at  its  anterior 
edge,  and  turned  backwards  so  as  to  expose  the  interior  of  the  cheek. 
On  the  mucous  membrane  will  be  seen  the  following : — 

1.  A  linear  series  of  small  papillne  opposite  the  upper  molar  teeth. 
Each  papilla  is  perforated  at  its  summit  by  a  duct  from  one  of  the 
lobules  of  the  superior  buccal  gland. 

2.  A  large  rounded  elevation  opposite  the  third  superior  molar. 
Stenson's  duct  opens  on  its  summit. 

3.  A  linear  series  of  small  papillso  opposite  the  inferior  molars.  Each 
papilla  is  perforated  by  a  duct  from  the  inferior  buccal  gland. 

The  Inferior  Buccal  Gland  will  be  exposed  by  incising  the  mucous 
membrane  along  the  last-mentioned  series  of  papilla?.  It  is  composed 
of  a  string  of  lobules  included  between  the  mucous  membrane  and  the 
buccinator. 

The  Buccal  Nerve  wdll  be  found  in  close  relation  to  the  inferior 
buccal  gland.  It  is  the  sensory  nerve  to  the  cheek,  and  is  derived  from 
the  inferior  maxillary  division  of  the  5th  cranial  nerve. 

THE  PTERYGO-MAXILLARY  REGION  AND  THE  REGION  OF  THE 
GUTTURAL  POUCH. 

Directions. — The  outer  surface  of  the  inferior  maxilla  having  been 
laid  bare  from  the  condyle  to  the  symphysis  by  the  removal  of  the 
muscles,  the  dissector  is  to  make  two  sections  with  the  saw.  The  first 
is  to  be  made  obliquely  from  a  jDoint  about  an  inch  above  the  last 
molar  tooth  to  the  angle  of  the  jaw ;  the  secOnd,  close  above  the 
symphysis.  In  making  the  sections,  the  edge  of  the  saw  must  be  kept 
parallel  to  the  surface  of  the  bone,  in  order  to  avoid  injury  to  the  sub- 
jacent structures.  The  jaw  is  next  to  be  disarticulated  by  inserting  a 
strong  scalpel  into  the  joint ;  and  the  scalpel  is  also  to  be  pjissed  round 
the  coronoid  process,  which  Avill  be  felt  embedded  in  the  temporal 
muscle,  in  the  temporal  fossa.  The  vertical  ramus  is  now  to  be  entirely 
removed,  at  the  same  time  leaving  m  position  the  parts  beneath  it. 
This  is  to  be  done  by  raising  the  bone  at  the  angle,  and  cutting  the 
muscular  fibres  inserted  into  its  deep  face.  The  edge  of  the  knife  is  t(» 
bo  kept  cutting  on  the  bone,  which  is  at  the  same  time  to  be  forcibly 
tilted  upwards  and  forwards  until  the  coronoid  process  is  torn  out  of 
the  temporal  muscle.  The  horizontal  ramus  is  next  to  be  folded  down- 
wards and  outwards ;  and  to  permit  this,  it  is  only  necessary  to  cut  and 
raise  slightly  the  mucous  membrane  below  the  molar  teeth.  When  these 
operations  have  been  effected,  the  dissection  will  take  the  form  of  Plate 
30.  The  vertical  ramus  is  to  be  retained  to  show  the  insertion  of  the 
pterygoid  and  temporal  muscles. 


DISSECTION    OF    THE    HEAD    AND    NECK.  185 

The  External  Pterygoid  Muscle  (Plate  30)  is  conical  in  form,  with 
the  apex  below.  Its  fibres  arise  from  the  outer  surface  of  the  sphenoid 
bone  above  the  entrance  to  the  subsphenoidal  canal,  and  from  the  entire 
outer  surface  of  the  subsphenoidal  jDrocess.  They  pass  upwards  and 
backwards  to  be  inserted  into  the  inner  aspect  of  the  neck,  or  constric- 
tion, below  the  condyle  of  the  lower  jaw. 

Action. — When  the  right  and  left  muscles  act  simultaneously,  the 
lower  incisors  are  made  to  protrude  in  front  of  the  upper.  When  the 
muscles  of  one  side  act  singly,  the  entire  inferior  maxilla  is  thrown  to 
the  opposite  side  ;  and  by  the  alternate  contraction  of  the  two  muscles, 
the  triturating  action  of  the  jaws  and  teeth  is  produced. 

The  Internal  Pterygoid  (Plate  30)  is  a  much  more  powerful  muscle 
than  the  preceding,  from  which  it  is  separated  by  the  inferior  dental 
nerve  and  vessels.  It  occupies  a  position  on  the  inside  of  the  vertical 
ramus  analagous  to  that  of  the  masseter  on  the  outside,  the  ramus 
being  included  between  the  two  muscles.  Its  fibres  arise  from  the 
bony  crest  formed  by  the  subsphenoidal  process  and  the  palatine  bone, 
and  they  pass  backwards  to  be  inserted  into  the  depressed  inner  surface 
of  the  vertical  ramus. 

Action. — The  principal  action  of  the  muscle  is  to  elevate  the  lower 
jaw  directly,  the  two  muscles  acting  together ;  but  acting  singly  and 
alternately,  they  assist  in  producing  lateral  movement. 

The  Temporal  Muscle  occupies  the  fossa  of  the  same  name.  Its 
fibres  arise  from  the  parietal,  frontal,  and  squamous  temporal  bones 
where  they  bound  this  fossa,  and  from  the  sphenoid  bone  above  the 
orbital  hiatus.  They  become  inserted  into  the  coronoid  process  and 
adjacent  part  of  the  anterior  border  of  the  vertical  ramus. 

Action. — Chiefly  to  elevate  the  lower  jaw,  acting  in  conjunction  with 
the  masseter  and  internal  pterygoid.  The  temporals  also  oppose  the 
action  by  which  the  external  pterygoids  protrude  the  lower  incisors  in 
front  of  the  upper. 

Directions. — Emerging  at  the  line  of  apposition  of  the  two  pterygoid 
muscles  are  the  inferior  dental  vessels  and  nerve,  the  mylo-hyoid  nerve, 
the  internal  maxillary  vein,  and  the  lingual  branch  of  the  5th  nerve. 
These  are  now  to  be  examined. 

The  Inferior  Dental  Artery  (Plate  30)  is  a  branch  of  the  internal 
maxillary,  detached  before  that  vessel  enters  the  subsphenoidal  canal. 
It  passes  first  between  the  two  pterygoid  muscles,  and  then  between  the 
inner  muscle  and  the  bone,  and  enters  the  inferior  dental  canal  of  the 
inferior  maxilla.  On  examining  the  inner  aspect  of  the  vertical  ramus, 
the  truncated  end  of  the  vessel  will  be  seen  at  the  upper  orifice  of  the 
canal.  It  is  continued  in  the  bone  beneath  the  molar,  canine,  and 
incisor  teeth,  supplying  these,  and  detaching  at  the  mental  foramen  a 
mental  branch. 


186 


THE  ANATOMY  OF  THE  HORSE. 


The  Inferior  Dental  Vein  (Plate  30)  has  a  distribution  similar  to 
that  of  the  artery  which  it  accompanies,  and  it  joins  the  internal 
maxillary  vein. 

The  Inferior  Dental  Nerve  (Plate  30)  is  a  branch  of  the  inferior 
maxillary  division  of  the  5th  nerve.  It  accompanies  the  vessels  in  the 
bone,  supplying  tlie  teeth,  and  detaching  sensory  mental  branches  to 
the  lower  lip. 

The  Mylo-Hyoid  Nerve  (Plate  30)  is  a  branch  of  the  preceding  nerve. 
It  descends  between  the  internal  pterygoid  muscle  and  the  bone,  and 
reaches  the  intermaxillary  space.  There  it  has  already  been  dissected 
on  the  surface  of  the  mylo-hyoid  muscle,  to  which  and  the  lower  belly  of 
the  digastricus  it  is  distributed. 

The  Lingual  or  Gustatory  Nerve  (Plate  30)  is  a  large  branch  of 
the  inferior  maxillary  division  of  the  5th.  Between  the  two  pterygoid 
muscles  it  is  joined  by  the  chorda  tympani  branch  of  the  7th  nerve, 
and  it  is  continued  between  the  internal  pterygoid  and  the  bone  to 
reach  the  tongue.     It  will  be  followed  in  the  dissection  of  that  organ. 

The  Internal  Maxillary  Vein  (Plate  30)  lies  in  front  of  the  pre- 
ceding nerve.  It  is  the  direct  continuation  of  the  buccal  vein  already 
dissected  in  the  cheek.  It  Avill  subsequently  be  followed  to  its  termina- 
tion, where  it  forms  the  jugular  by  union  with  the  superficial  temporal 
vein. 

The  Lingual  Vein  (Plate  30)  drains  blood  away  from  the  tongue. 
It  runs  in  company  with  the  gustatory  nerve;  and  after  receiving 
branches  from  the  soft  palate  and  the  pharynx,  it  joins  the  buccal  vein 
between  the  internal  pterygoid  muscle  and  the  bone. 

Pterygoid  Vessels.  The  pterygoid  muscles  derive  their  blood  from 
branches  of  the  internal  maxillary  or  of  the  inferior  dental  artery. 
The  inner  muscle  receives  also  the  inner  division  of  the  maxillo-mus- 
cular  artery.     The  pterj'goid  veins  join  the  internal  maxillary  vein. 

Directions. — The  external  and  internal  pterygoid  muscles  must  now 
be  removed.  In  cutting  away  the  outer  muscle,  the  thick  buccal  nerve 
will  be  found  passing  through  it  near  its  origin,  and  giving  branches  to 
the  muscle.  The  nerve  is  to  be  preserved  to  show  its  origin.  Care 
must  also  be  taken  of  the  internal  maxillary  vessels,  and  of  the  inferior 
maxillary  nerves,  which  are  included  between  the  muscle  and  the 
guttural  pouch.  The  internal  pterygoid  is  to  be  cut  close  to  its  origin, 
and  taken  away  without  severing  the  vessels  and  nerves  just  dissected. 
The  pterygoid  branch  of  the  inferior  maxillary  division  of  the  5th  nerve 
will  be  found  entering  it,  and  an  inch  or  two  of  the  nerve  should  be 
preserved  to  show  its  origin.  Extending  along  the  deep  aspect  of  the 
origin  of  the  muscle  is  the  tensor  palati  muscle,  and  care  must  be  taken 
not  to  injure  it.  The  deep  face  of  the  internal  pterygoid  is  related  to 
the  phai'ynx,  the  guttural  pouch,  the  great  cornu  of  the  hyoid  bone,  tlic 


DISSECTION    OF    THE    HEAD    AND    NECK.  187 

intermediate  tendon  of  the  digastricus,  the  larynx,  and  the  submaxillary 
gland.  Besides  these,  it  is  related  to  Stenson's  duct  and  the  submaxillary 
vessels,  which  have  already  been  dissected  on  its  surface  in  the  inter- 
maxillary space. 

The  Guttural  Pouches  (Plates  31  and  32).  There  are  two  large 
cavities  situated  at  the  base  of  the  skull,  above  the  pharynx,  and  be- 
tween the  great  (styloid)  cornua  of  the  hyoid  bone.  Anteriorly  they 
extend  as  far  as  the  upper  margin  of  the  posterior  nares,  and  posteriorly 
as  far  as  the  atlanto-axial  articulation.  Inwardly  the  mucous  lining  of 
the  two  pouches  forms  a  mesial  partition.  Outwardly  each  pouch  has 
numerous  relations,  the  chief  of  which  are  as  follows  : — Behind  the 
great  cornu  the  pouch  is  covered  by  the  submaxillary  gland,  and  the 
stylo-maxillaris,  digastricus  (upper  belly),  stylo-hyoid,  and  occipito- 
styloid  muscles,  and  is  crossed  by  the  external  carotid,  internal  carotid, 
and  occipital  arteries,  and  by  the  9th,  10th,  11th,  12th,  and  sympa- 
thetic nerves.  In  front  of  the  great  cornu.  the  pouch  is  covered  by  the 
parotid  gland  and  the  internal  pterygoid  muscle,  and  is  crossed  by  the 
internal  maxillary  vessels,  the  chorda  tympani  nerve,  and  the  inferior 
maxillary  division  of  the  5th  nerve.  The  pouch  is  lined  by  mucous 
membrane  continuous  with  that  of  the  Eustachian  tube,  and  by  that 
tube  it  communicates  with  the  pharynx  and  the  middle  ear.  Normally 
the  pouch  contains  air,  which  it  receives  from  the  pharynx  through  the 
Eustachian  tube.  When  the  mucous  lining  of  the  pouch  becomes 
inflamed,  pus  tends  to  accumulate  in  the  cavity,  since  the  Eustachian 
orifice,  by  which  the  inflammatory  products  might  escape  into  the 
pharynx,  is,  in  the  ordinary  position  of  the  head,  situated  towards  the 
ujDper  part  of  the  pouch. 

The  Eustachian  Tube  (Plate  32).  This  is  a  fibro-cartilaginous  tube 
of  three  or  four  inches  in  length,  extending  downwards  from  the  petrous 
temporal  bone  to  the  pharynx.  At  its  upper  extremity  the  tube  com- 
municates with  the  cavity  of  the  middle  ear  (in  the  temporal  bone),  and 
at  its  lower  extremity  it  opens  into  the  pharynx  by  a  slit-like  aper- 
ture. For  nearly  the  whole  of  its  extent  the  tube  is  slit  open  along 
its  outer  side,  and  is  thiis  in  free  commvmication  with  the  guttural 
pouch.  The  tube  is  lined  b}'  mucous  membrane,  and  through  its  agency 
air  is  admitted  from  the  pharynx  to  the  guttural  pouch  and  the  middle 
car. 

Tlie  Internal  Maxillary  Artery  (Plates  31  and  32)  results  from  the 
division  of  the  external  carotid  on  the  outer  surface  of  the  great  cornu 
of  the  hyoid  bone.  It  is  much  larger  than  the  superficial  temporal, 
which  is  the  other  terminal  branch  of  the  external  carotid.  In  passing  to 
enter  the  subsphenoidal  canal,  it  describes  a  double  or  sigmoid  curve, — 
the  first  convex  downwards,  the  second  upwards.  In  this  course  the 
artery  is  placed   within  the   articulation   of  the  jaw  and  the  external 


188  THE    ANATOMY    OF    THE    HORSE. 

pterygoid  muscle,  and  rests  s\iccessively  on  the  guttural  pouch  and  the 
tensor  palati  muscle.  It  is  crossed  superficially  by  the  inferior  dental 
and  lingual  nerves,  and  deeply  by  the  clioi-da  tynipani.  It  detaches  the 
following  collateral  branches  : — 

1.  The  Inferior  Dental  Artery,  a  large  branch  arising  from  the  convexity 
of  the  first  curve.  It  has  already  been  seen  entering  the  inferior  dental 
canal. 

2.  The  Tympanic  Artery,  the  smallest  of  the  branches  here  given  off, 
is  detached  at  nearly  the  same  point  as  the  preceding,  but  from  the 
opposite  side  of  the  parent  vessel.  It  lies  beside  the  chorda  tympani 
nerve,  and  penetrates  the  petrous  temporal  bone  to  be  distributed  in  the 
tympanum,  or  middle  ear. 

3.  The  Great  Meningeal  (spheno-spinous)  Artery,  a  vessel  of  variable 
volume  detached  from  the  upper  side  of  the  parent  artery  beneath  the 
inferior  maxillary  nerve,  and  entering  the  cranial  cavity  by  the  foramen 
lacerum  basis  cranii. 

4.  The  Pterygoid  Arteries,  two  or  three,  arising  from  the  concavity  of 
the  secotad  curve. 

5.  The  Posterior  Deep  Temporal  Artery,  given  ofi'  from  the  upper  side 
of  the  parent  trunk  about  half  an  inch  before  it  enters  the  subsphen- 
oidal  canal.  It  enters  the  temporal  muscle  by  passing  in  front  of  the 
condyle  of  the  temporal  articular  surface,  and  it  communicates  with  the 
masseteric  division  of  the  transverse  facial  artery  by  a  slender  branch 
which  passes  through  the  corono-cond^doid  notch. 

The  Internal  Maxillary  Vein  passes  between  the  two  pterygoid 
muscles ;  and  crossing  below  the  articulation  of  the  jaw,  it  joins  the 
supei-ficial  temporal  vein  to  form  the  jugular.  The  junction  takes  place 
in  the  substance  of  the  parotid,  a  few  lobules  of  the  gland  being  inter- 
posed between  it  and  the  termination  of  the  external  carotid  arteiy. 

The  Inferior  Maxillary  Division  of  the  5th  Nerve  (Plate  31). 
This  is  a  thick  cord  containing  both  sensory  and  motor  filaments  which 
issues  from  the  cranium  by  the  forepart  of  the  foramen  lacerum  basis 
cranii.  It  passes  obliquely  downwards  and  backwards  on  the  wall  of  the 
guttural  pouch,  in  front  of  the  temporo-hyoideal  articulation,  and  divides, 
about  an  inch  below  its  point  of  exit,  into  two  branches  of  nearly  equal 
size — the  inferior  dental  and  lingual  nerves.  As  it  issues  from  the 
foramen,  it  gives  off  the  following  branches  : — 

1.  The  Stihzygomatic  Nerve  is  detached  from  the  posterior  aspect  of 
the  trunL  It  turns  round  behind  the  ai'ticulation  of  the  jaw,  and  has 
already  been  seen  to  join  the  7th  nerve,  which  it  accompanies  in  its 
distribution  on  the  face.  It  sends  a  branch  in  company  with  the  trans- 
verse facial  vessels. 

2.  The  Nerve  to  the  Internal  Pterygoid  arises  from  the  antero-inferior 
aspect  of  the  trunk. 


DISSECTION    OF    THE    HEAD    AND    NECK.  189 

3.  The  Nerve  to  the  Masseter  and  Temporal  Muscles.  This  is  given  off 
from  the  front  of  the  trunk.  It  detaches  branches  to  the  temporal 
muscle,  and  is  then  continued  through  the  corono-condyloid  notch  to 
end  in  the  masseter. 

4.  The  Buccal  Nerve  is  a  larger  branch  than  any  of  the  foregoing. 
It  arises  at  the  same  point  as  the  preceding  nerve,  and  passing  through 
the  external  pterygoid  muscle,  to  which  and  the  temporal  muscle  it 
supplies  branches,  it  is  continued  as  a  sensory  nerve  to  the  cheek. 

The  Inferior  Dental  Nerve  is  the  larger  of  the  two  branches  into 
which  the  trunk  of  the  inferior  maxillary  nerve  divides.  Under  cover  of 
the  external  pterygoid  muscle,  it  crosses  over  the  internal  maxillary  artery, 
in  company  with  the  lingual  nerve,  which  lies  in  front  of  it.  It  then 
passes  in  between  the  two  pterygoid  muscles,  where  it  places  itself  in 
front  of  the  inferior  dental  vessels.  It  has  already  been  seen  issuing 
from  between  the  muscles  to  gain  the  inferior  dental  canal  by  passing 
between  the  inner  muscle  and  the  bone. 

The  Mylo-hyoid  Nerve  is  detached  from  the  posterior  edge  of  the  fore- 
going nerve  at  its  point  of  formation.  It  crosses  the  inferior  dental 
vessels  between  the  two  pterygoid  muscles,  and  then  descends  between 
the  inner  muscle  and  the  bone.  It  has  already  been  traced  to  the 
digastric  (lower  belly)  and  mylo-hyoid  muscles. 

The  Lingual  or  Gustatory  Nerve  is  only  slightly  smaller  than  the 
inferior  dental.  It  lies  immediately  in  front  of  that  nerve  as  far  as  the 
posterior  edge  of  the  external  pterygoid  muscle,  where  it  passes  forwards 
between  the  internal  pteiygoid  and  the  bone  to  reach  the  tongue.  It  is 
a  sensory  branch ;  and,  while  between  the  two  muscles,  it  is  joined  by 
the  chorda  tympani. 

The  Chorda  Tympani  Nerve.  This  is  a  branch  detached  from  the 
7th  nerve  in  the  aqueduct  of  Fallopius.  It  passes  across  the 
tympanum,  or  middle  ear,  and  issues  from  the  petrous  temporal  bone 
by  the  styloid  foramen.  It  then  descends  on  the  guttural  pouch ;  and 
crossing  beneath  the  internal  maxillary  artery  and  the  trunk  of  the 
inferior  dental  nerve,  it  joins  the  lingiial  nerve  between  the  two 
pterygoid  muscles. 

The  Otic  Ganglion.  This  ganglion  is  at  best  minute,  and  sometimes 
absent,  or  at  least  not  well  defined.  To  examine  it  well,  it  is  neces- 
sary to  make  a  special  preparation,  exposing  the  inner  aspect  of  the 
inferior  maxillary  nerve  at  its  point  of  emergence  from  the  cranium. 
The  ganglion  is  placed  on  the  inner  side  of  the  before-mentioned  nerve- 
trunk  near  the  origin  of  its  buccal  branch.  Its  afferent  branches  are  : 
(1)  twigs  from  the  buccal  branch  of  the  inferior  maxillary  nerve  ;  (2)  the 
small  supei-ficial  petrosal  nerve  from  the  7th ;  (3)  twigs  from  the  sympa- 
thetic branches  that  accompany  the  internal  maxillary  artery.  It 
supplies    efferent    branches    to:     (1)  the    tensor   palati    muscle;     (2) 


190  THE  ANATOMY  OF  THE  HORSE. 

tlic  tensor  tympani ;  (3)  the  pterygoid  muscles ;  (4)  the  Eustachian 
tube. 

The  Submaxillary  Gland  (Plates  27  and  31)  is,  in  point  of  size,  the 
second  of  the  salivary  glands.  It  is  elongated,  with  blunt,  rounded  extremi- 
ties ;  and  it  is  curved,  the  concavity  being  directed  upwards  and  forwards. 
Its  outer  surface  is  related  to  the  tendon  of  the  sterno-maxillaris  nuiscle, 
to  the  fibrous  band  connecting  that  tendon  to  the  mastoid  insertion  of 
the  mastoido-humeralis,  and  to  the  internal  pterygoid.  The  sterno- 
maxillaris  tendon  and  the  above-mentioned  fibrous  band  separate  it  from 
the  overlying  parotid.  Its  imier  surface  is  related  to  the  guttural 
pouch,  the  larynx,  and  the  thyro-hyoid  muscle ;  and  it  conceals  above 
the  larynx  the  terminal  part  of  the  common  carotid  artery,  and  the 
10th  and  11th  nerves.  Its  posterior  border  is,  about  its  centre,  near 
or  in  contact  with  the  thyroid  gland ;  and  below  that  point  it  is 
margined  by  the  submaxillary  vein.  Its  anterior  border  is  related 
above  to  the  stylo-maxillaris  muscle,  and  for  the  rest  of  its  extent  it  is 
traversed  by  Wharton's  duct — the  excretory  duct  of  the  gland.  The 
superior  extremity  of  the  gland  is  loosely  maintained  beneath  the  wing 
of  the  atlas ;  the  inferior  extremity  is  situated  within  the  intermaxillary 
space,  and  is  crossed  o\itwardly  by  the  submaxillary  artery. 

Wharton's  Duct  is  formed  by  the  union  of  small  branches  which 
emanate  from  the  gland  structure  along  its  anterior  border.  It  descends 
along  that  border,  and  at  the  lower  extremity  of  the  gland  it  crosses  to 
the  inner  side  of  the  submaxillary  artery,  and  is  continued  beneath  the 
tongue,  where  it  will  subsequently  be  followed  in  its  course  towards  tlic 
harh. 

The  Digastricus  (Plate  31).  The  iipper  belly  of  this  muscle  has 
already  been  seen  to  arise  from  tlie  styloid  process  of  the  occipital  bone, 
where  it  is  confounded  witli  the  stylo-maxillaris  muscle.  It  is  succeeded 
by  an  intermediate  tendon,  which  plays  throiigh  a  perforation  in  the 
tendon  of  the  stylo-hyoid  nmscle,  in  front  of  which  it  joins  the  lower 
belly.  The  lower  belly  has  already  been  dissected  in  the  intermaxillary 
space,  where  it  is  inserte.d  by  tendinous  slips  into  the  posterior  border  of 
the  inferior  maxilla,  above  the  symphysis. 

Action. — To  depress  the  lower  jaw. 

The  Stylo-hyoid  (Plate  31).  This  nmscle  arises  from  the  extreme 
upper  part  of  the  hinder  edge  of  the  great  cornu.  Its  inferior  tendon  is 
perforated  for  the  passage  of  the  digastricus,  and  is  inserted  into  the  base 
of  the  thyroid  cornu  of  the  hyoid  bone.  The  external  carotid  emerges 
between  the  belly  of  the  muscle  and  the  great  cornu. 

Action, — It  carries  the  base  of  the  tongue  and  the  larynx  upwards  and 
backwards,  by  flexing  the  joint  between  the  great  and  small  cornua,  and 
the  joint  between  the  small  cornu  and  the  body. 

Directions. — Pin  tlie  lower  extremity  of  the  submaxillary  gland  and 


DISSECTION    OF    THE    HEAD    AND    NECK.  191 

Wharton's  duct  in  position,  and  then  remove  the  remainder  of  the  gland 
together  with  the  stylo-hyoid,  the  digastricus,  and  the  remains  of  the 
stylo-maxillaris.  This  will  expose  the  posterior  part  of  the  guttural 
pouch,  the  pharynx,  the  larynx,  the  pharyngeal  lymphatic  glands,  the 
upper  part  of  the  external  carotid  artery  (and  its  terminal  branches — the 
external  and  internal  carotids,  and  the  occipital),  the  superior  cervical 
ganglion  of  the  sympathetic  (and  the  upper  part  of  the  cervical  cord), 
the  first  parts  of  the  9th,  10th,  11th,  and  12th  cranial  nerves,  and  the 
inferior  primary  branch  of  the  1st  cervical  nerve. 

Pharyngeal  Lymphatic  Glands.  These  form  an  elongated  cluster 
situated  at  the  upper  part  of  the  side  of  the  pharynx.  They  are  placed 
on  the  course  of  all  the  lymphatic  vessels  of  the  head. 

The  Common  Carotid  Artery  (Plate  32)  divides  above  the  cricoid 
cartilage  of  the  larynx,  and  under  cover  of  the  submaxillary  gland 
or  the  stylo-maxillaris  muscle,  into  three  branches,  viz.,  external 
carotid,  internal  carotid,  and  occipital  arteries.  The  first  of  these  con- 
tinues the  direction  of  the  parent  trunk,  and  is  much  larger  than  either 
of  the  others,  which  are  of  nearly  equal  size. 

The  Occipital  Artery  (Plate  32).  The  root  of  this  vessel  is  external 
to,  and  slightly  in  advance  of,  the  root  of  the  internal  carotid.  It  passes 
upwards  and  slightly  forwards  over  the  anterior  straight  muscles  of  the 
head,  and  enters  the  antero-external  foramen  of  the  atlas.  In  the 
groove  which  connects  this  and  the  antero-internal  foramen,  the  artery 
divides  into  occipito-muscular,  and  cerebro-spinal  branches.  Before  its 
passage  through  the  foramen  the  vessel  detaches  three  collateral 
branches,  viz.,  prevertebral,  mastoid,  and  retrograde  or  anastomotic 
arteries. 

1.  The  Prevertebral  Artery. — This  is  the  first  and  most  slender  of  the 
three  branches.  Passing  upwards  and  forwards,  it  supplies  muscular 
twigs  to  the  anterior  straight  muscles  of  the  head,  and  oneningeal  twigs 
that  pass  into  the  cranium  by  the  foramen  lacerum  basis  cranii  and  the 
condyloid  foramen. 

2.  The  Mastoid  Artery,  a  considerable  vessel,  is  detached  about  one- 
third  of  an  inch  above  the  preceding.  It  crosses  over  the  edge  of  the 
rectus  capitis  lateralis,  and  ascends  behind  the  styloid  process  of  the 
occipital  bone,  where  it  has  already  been  exposed  (page  170). 

3.  The  Retrograde  or  Anastomotic  Branch  varies  considerably  in 
volume.  Arising  between  the  obliquus  capitis  superior  and  the  rectus 
capitis  latei'alis,  beneath  the  wing  of  the  atlas,  it  passes  backwards 
through  the  posterior  alar  foramen,  and  anastomoses  with  the  termina- 
tion of  the  vertebral  artery. 

The  Internal  Carotid  Artery  (Plate  32)  is  a  long  vessel  which  is 
the  main  source  of  supply  to  the  brain.  It  passes  obliquely  upwards 
and  forwards,  supported  by  the  membrane  of  the  guttural  pouch,  and 


192  THE  ANATOMV  OF  THE  HORSE. 

enters  the  cranium  by  the  foramen  lacerum  basis  cranii.  Its  mode  of 
entrance  and  its  distribution  to  the  brain  will  be  examined  at  a  later 
stage. 

The  External  Carotid  Artery  (Plates  31  and  32).  This  vessel  may, 
for  the  purposes  of  description,  be  divided  into  two  portions.  The  first 
portion,  comprising  two-thirds  of  the  artery,  is  included  between  the  gut- 
tural pouch  inwardly,  and  the  stylo-maxillaris,  digastric,  and  stylo-hyoid 
muscles  outwardly.  It  emerges  from  beneath  the  last-named  muscle, 
and  joins  the  second  poi'tion  by  passing  between  the  muscle  and  the 
posterior  edge  of  the  great  cornu  of  the  hyoid  bone.  It  is  crossed 
inwardly  by  the  9th,  and  outwardly  by  the  12th  nerve.  The  second 
portion  passes  obliquely  upwards  and  forwards  on  the  great  cornu,  at 
the  upper  part  of  which,  and  under  cover  of  the  parotid  gland,  it  divides 
into  the  superficial  temporal  and  internal  maxillary  arteries.  The 
collateral  branches  of  the  vessel  are :  the  submaxillary,  maxillo-muscular, 
and  posterior  auricular  arteries.  The  last  two  spring  from  the  second 
portion  of  the  artery,  and  have  already  been  dissected. 

The  Submaxillary  or  Facial  Artery  is  detached  from  the 
external  carotid  beneath  the  digastric  muscle.  It  is  a  vessel  of  large 
calibre,  being  nearly  equal  to  the  parent  vessel  beyond  its  point  of 
detachment.  It  descends  over  the  pharynx,  being  nearly  parallel  to  the 
posterior  edge  of  the  great  cornu,  and  about  an  inch  behind  it.  At  first 
under  cover  of  the  digastric  and  stylo-hyoid  muscles,  it  then  turns 
round  the  anterior  edge  and  outer  surface  of  the  stylo-hyoid  where  the 
intermediate  tendon  of  the  digastric  plays  through  it.  Continuing  its 
course,  it  crosses  Wharton's  duct  and  the  lower  extremity  of  the  sub- 
maxillary gland  to  the  outer  side,  and  appears  in  the  intermaxillary 
space  between  the  internal  pterygoid  and  subscapulo-hyoid  muscles. 
Its  further  course  in  the  intermaxillary  space  and  on  the  face  has  already 
been  followed.  Behind  the  great  cornu  the  artery  is  in  company  with 
the  12th  nerve.  From  its  origin  to  the  extremity  of  the  submaxillary 
gland,  it  detaches  three  collateral  branches,  viz.,  the  pharyngeal,  lingual, 
and  submental  arteries. 

1.  The  Pharyngeal  Artery  is  a  small  branch  given  off  at  the  anterior 
edge  of  the  stylo-hyoid  or  under  cover  of  that  muscle.  It  reaches  the 
pharynx  by  passing  beneath  the  great  cornu,  crossing  either  outwardly 
or  inwardly  the  9th  nerve.  It  is  distributed  to  the  pharynx,  giving  also 
a  forward  l3ranch  to  the  soft  palate. 

2.  The  Linr/ual  Artery^  whose  volume  is  about  equal  to  that  of  the 
distal  part  of  the  parent  trunk,  has  its  point  of  origin  over  the  tip  of 
the  thyroid  cornu.  It  descends  on  the  cerato-hyoid  muscle,  and  reaches 
the  tongue  by  passing  beneath  the  great  hyo-glossus  muscle.  It  is  the 
main  vessel  of  supply  to  the  tongue,  and  will  be  followed  in  the  dissec- 
tion of  that  organ. 


DISSECTION    OP   THE    HEAD   AND   NECK.  193 

3.  The  Submental  Artery  is  detached  at  the  extremity  of  the  sub- 
maxillary gland,  as  the  parent  artery  appears  in  the  intermaxillary 
space.     It  has  already  been  seen  in  the  dissection  of  that  space. 

The  Occipital  Vein.  This  vein  descends  from  beneath  the  wing  of 
the  atlas,  in  company  with  the  artery  of  the  same  name  ;  and  joins  the 
juguhxr  at  the  posterior  edge  of  the  parotid  gland,  a  little  above  the 
termination  of  the  submaxillary  vein.  It  is  formed  by  the  union  of 
branches  corresponding  to  those  of  the  artery;  and,  besides  these,  it 
receives  a  branch  which  comes  from  the  spinal  canal  by  traversing  the 
ring  of  the  atlas,  beneath  the  wing,  and  another  from  the  subsphenoidal 
sinus.     This  last  will  be  exposed  at  a  later  stage. 

The  9th  Cranial  Nerve,  also  called  the  Glosso-Pharyngeal  (Plate 
32),  issues  from  the  cranium  by  the  posterior  part  of  the  foramen  lacerum 
basis  cranii.  It  descends  on  the  guttural  pouch,  behind  the  great  cornu 
of  the  hyoid  bone,  and  under  cover  of  the  digastric  and  stylo-hyoid 
muscles ;  and  it  here  crosses  to  the  inner  side  of  the  external  carotid 
artery.  Reaching  the  pharynx,  it  continues  to  descend  either  close 
behind  the  posterior  edge  of  the  great  cornu  or  under  cover  of  it ;  and 
here  it  is  crossed  by  the  pharyngeal  artery,  which  may  pass  either  over 
or  under  it.  It  next  passes  within  the  articulation  of  the  great  and 
small  cornua  to  reach  the  root  of  the  tongue,  where  its  terminal  branches 
will  be  seen  at  a  later  stage.  It  gives  off  the  following  branches,  the 
first  of  which  will  not  now  be  seen  : — 

1.  The  Nerve  of  Jacohson,  given  off  from  Andersch's  ganglion — a 
minute  ganglion  placed  on  the  nerve  where  it  issues  from  the  cranium. 
Jacobson's  nerve  enters  a  minute  foramen  in  the  petrous  temporal  bone, 
and  is  distributed  to  the  tympanum. 

2.  Branches  of  communication  with  the  superior  cervical  ganglion  of 
the  sympathetic. 

3.  A  Branch  to  the  Carotid  Plexus. 

4.  A  Pharyngeal  branch,  which  is  given  off  at  or  near  the  point  where 
the  nerve  crosses  the  external  carotid.  It  is  as  large  as  the  glossal 
continuation  of  the  trunk,  and  it  passes  on  to  the  wall  of  the  pharynx, 
behind  the  stylo-pharyngeus  muscle. 

5.  A  Branch  to  the  Stylo-pliaryngeus  muscle. — This  may  be  detached 
either  before  or  after  the  preceding  branch.  It  enters  the  outer  side  of 
the  muscle. 

The  12th  Cranial  Nerve,  called  also  the  Hypoglossal  (Plate 
32),  leaves  the  cranium  by  the  condyloid  foramen.  It  passes  through 
the  angle  of  separation  of  the  10th  and  11th  nerves,  and  descends  on 
the  guttural  pouch,  crossing  to  the  outer  side  of  the  external  carotid  at 
or  near  the  origin  of  the  submaxillary  artery.  It  next  crosses  the 
phaiynx  in  company  with  the  submaxillary  artery,  and  passes  under  the 
angle  formed  by  that  vessel  and  its  lingual  branch.     It  is  continued  to 


194  THE  ANATOMY  OF  THE  HORSE. 

the  muscles  of  the  tougixe.  Where  the  nerve  lies  on  the  guttural  pouch, 
it  is  covered  by  the  stylo-maxillaris,  digastric,  and  stylo-liyoid  muscles, 
and  below  that  point  it  is  covered  by  the  internal  pterygoid  nuiscle. 

On  the  pharynx  tlie  hypoglossal  is  joined  by  a  considerable  twig  from 
the  inferior  primary  branch  of  the  1st  spinal  nerve.  On  the  guttural 
pouch  it  constantly  communicates  with  the  superior  cervical  ganglion  of 
the  sympathetic.      It  has  no  other  branches  until  it  reaches  the  tongue. 

The  10th  Cranial  Nerve,  also  termed  the  Vagus  or  Pneumogastric 
(Plate  32),  issues  from  the  cranium  by  the  extreme  posterior  part  of  the 
foramen  lacerum  basis  cranii.  For  about  one  inch  and  a  half  of  its 
course  it  forms  a  common  cord  with  the  11th  nerve,  which  issues  at  the 
same  point  The  two  nerves  then  separate  (the  12th  nerve  passing 
through  the  angle),  and  the  vagus  passes  downwards  and  backwards  on 
the  guttural  pouch.  It  passes  over  the  internal  carotid  artery,  and 
imder  the  occipital ;  and  above  the  first  part  of  the  trachea  it  meets  the 
cervical  cord  of  the  sympathetic,  the  two  nerves  then  uniting  to  form  a 
common  cord,  which  applies  itself  to  the  \ipper  side  of  the  common 
carotid  artery,  and  descends  with  it  in  the  neck.  Between  the  foramen 
lacerum  and  the  point  where  the  nerve  joins  the  sympathetic,  it  detaches 
the  following  branches  : — 

1.  Branches  of  Communication  with  the  superior  cei-vical  ganglion. 

2.  A  Pharywjtal  Branch  is  detached  near  the  point  where  the  vagus 
passes  under  the  occipital  artery.  It  passes  to  the  inner  side  of  the 
external  carotid  artery,  and  reaches  the  pharynx.  There  it  unites  with 
the  sympathetic  and  the  pharyngeal  branch  of  the  9th,  forming  a  plexus 
from  which  branches  pass  to  the  constrictors  and  mucous  membrane  of 
tlie  pharynx,  and  to  the  first  part  of  the  ocsopliagus. 

3.  The  Superior  Laryngeal  Nerve  is  given  off  near  the  termination  of 
the  common  carotid  artery ;  and  crossing  beneath  the  external  carotid 
or  the  termination  of  the  common  carotid,  it  passes  over  the  pharynx  to 
penetrate  the  thyroid  cartilage  at  the  anterior  edge  of  the  thyro- 
pharyngeus  muscle.  Within  the  larynx,  as  will  afterwards  be  learned,  it 
is  distributed  to  the  mucous  membrane  ;  and  also  gives  branches  to  the 
pharynx,  oesophagus,  and  root  of  the  tongue.  Near  its  origin  it  detaches 
an  external  laryngeal  Wanch,  which  passes  to  the  crico-thyroid  and  crico- 
pharyngeus  muscles. 

The  trunk  of  the  vagus  is  sometimes  distinctly  gangliform  at  the 
point  of  detachment  of  its  superior  laryngeal  branch.  This  is  the 
ganglion  of  the  trunk  of  human  anatomy. 

The  11th  Cranial  Nerve,  called  also  the  Spinal  Accessory  Nerve, 
(Plate  32),  issues  from  the  cranium  by  the  posterior  part  of  the  foramen 
lacerum,  in  company  with  the  vagus.  For  the  space  of  about  one  inch 
and  a  half  it  forms  a  common  cord  with  that  nerve.  It  then  parts 
oompany  with  the  vagus,  and  passes  backwards  at  the  edge  of  the  rectus 


DISSECTION    OF    THE    HEAD    AND    NECK.  195 

capitis  anticus  major  muscle,  where  it  is  crossed  superfcially  by  the 
occipital  artery.  It  then  turns  round  the  wing  of  the  atlas  at  its  most 
prominent  point ;  and  passes  beneath  the  mastoido-humeralis  muscle, 
crossing  the  branches  of  the  2nd  spinal  nerve.  Before  it  disappears 
beneath  the  mastoido-humeralis,  it  comnnmicates  with  the  superior 
cervical  ganglion,  and  gives  a  branch  to  the  sterno-maxillaris  muscle. 
Its  distribution  in  the  neck  has  already  been  followed. 

The  Sympathetic  Nervk  (Plate  32).  The  initial  part  of  the  cervical 
cord  is  here  seen  passing  back  from  the  superior  cervical  ganglion. 
After  a  course  of  a  few  inches  it  places  itself  beside  the  vagus,  and  forms 
a  common  cord  with  it. 

The  SuPERioK  Cervical  Ganglion  is  placed  on  the  guttural  pouch, 
above  the  internal  carotid  artery.  It  is  about  half  an  inch  in  length, 
fusiform  in  shape,  and  of  a  reddish-grey  colour.  Below  it  tapers  into 
the  cervical  cord,  and  above  it  tapers  into  the  ascending  oflFsets  with  the 
internal  carotid  artery.  Connecting  branches  unite  the  ganglion  with 
the  1st  spinal  nerve,  and  with  the  last  four  cranial  nerves.  Communi- 
cations with  some  of  the  other  cranial  nerves  are  also  established  through 
the  carotid  offsets  of  the  ganglion.  The  distributory  branches  from  the 
ganglion  pass  to  the  adjoining  vessels  and  the  pharynx.  Of  the  former 
set,  two  branches  accompany  the  internal  carotid  into  the  skull,  and  form 
the  carotid  and  cavernoiis  plexuses.  Other  branches  follow  the  external 
carotid,  and  are  continued  on  the  branches  of  that  artery. 

The  1st  Cervical  Nerve.  The  inferior  primary  branch  of  this 
nerve  descends  through  the  antero-external  foramen  of  the  atlas,  and 
appears  in  company  with  the  occipital  vessels,  between  the  rectus  capitis 
lateralis  and  the  obliquus  capitis  superior.  It  passes  towards  the  upper 
extremity  of  the  trachea,  and  splits  into  branches  that  enter  the 
terminal  parts  of  the  subscapulo-hyoid,  sterno-hyoid,  and  sterno- 
thjToid  muscles.  Beneath  the  atlas  it  gives  branches  to  the  anterior 
and  lateral  straight  muscles  of  the  head ;  and  beyond  that  point  it 
furnishes  a  branch  which  supplies  the  thyro-hyoid  muscle,  and  gives  a 
twig  to  join  the  hypoglossal  nerve.  It  sends  a  communicating  branch 
to  the  superior  cervical  ganglion  of  the  sympathetic. 

the  tongue. 

Directions. — With  the  cavity  of  the  mouth  exposed  as  in  Plate  30, 
any  poi-tions  of  food  found  in  the  cavity  should  be  removed,  and  the 
mucous  membrane  made  clean.  By  moving  the  tongue  about,  the 
following  points  will  be  observed. 

The  inferior  portion  of  the  tongue  lies  free  on  the  floor  of  mouth,  from 
which  it  can  readily  be  drawn  out.  This  part  is  two-sided,  being  flat- 
tened from  before  to  behind,  and  rounded  at  its  extremity  like  a  spatula. 
The  superior  portion,  on  the  other  hand,  cannot  be   displaced,  and  is 


196  THE  ANATOMY  OF  THE  HOUSE. 

thick  and  of  a  three-sided  fomi.  The  entire  organ  is  invested  by  the 
mucous  membrane  of  the  mouth,  which,  in  passing  on  to  the  tongue, 
forms  certain  folds,  or  doublings.  Thus,  if  the  free  portion  of  the  tongue 
be  raised  from  its  position  on  the  floor  of  the  mouth,  there  will  be  seen 
on  the  middle  line  a  double  fold  of  mucous  membrane  termed  the  frcenum 
lingiue.  At  the  extreme  upper  part  of  the  mouth,  again,  the  mucous  mem- 
brane, in  passing  between  the  root  of  the  tongue  and  the  soft  palate, 
forms  on  each  side  a  fold  termed  the  anteinor  jnllar  of  the  fauces. 

The  mucous  memlirane  of  the  tongue  has  its  surface  raised  into  the 
form  of  papillae  of  which  there  are  three  varieties  : — 

1.  The  Filiform  Papillce  are  the  most  numerous,  and  are  found  all 
over  the  tongue.  In  shape  they  are  conical,  having  a  tapering  summit 
either  simple  or  bearing  secondary  papillae.  They  are  largest  on  the 
upper  half  of  the  dorsum  (anterior  or  upper  surface),  to  which  they 
give  a  distinct  pile. 

2.  The  Fungifmm  Papillce  are  mushroom-shaped,  being  expanded  at 
the  summit,  which  bears  secondary  papillse.  They  are  scattered  along 
the  doi-sum  and  sides  of  the  tongue,  being  most  numerous  in  its  middle 
portion. 

3.  The  Circumvallate  Papillce  are  generally  two  in  number,  and  are 
placed  on  the  dorsum,  one  on  each  side  of  the  middle  line,  aboiit  five 
inches  from  the  epiglottis.  Sometimes  there  is  a  third  and  smaller 
papilla,  placed  on  the  middle  line,  about  three-quarters  of  an  inch  above 
the  other  two.  Each  is  isolated  by  a  circular  trench,  and  is  terminated 
by  a  flat  summit,  Avhich  is  level  with  the  sun-ounding  surface,  and 
bears  numerous  secondary  papillee.  They  contain  the  peculiar  gxtstatory 
bodies,  to  which  fibres  of  the  glosso-pharyngeal  nerve  are  distributed. 

The  Sublingual  Ridge. — This  is  a  longitudinal  elevation  of  the  mucous 
membrane  at  the  floor  of  the  mouth,  on  each  side  of  the  fixed  portion  of 
the  tongue.  It  is  caused  by  the  underlying  sublingual  salivary  gland, 
whose  ducts  open  on  the  summits  of  little  tubercles  which  stud  the 
ridge. 

The  Barbs. — These  are  two  flattened,  leaf-like  papilla)  situated  on  the 
inferior  part  of  the  floor  of  the  mouth,  one  on  each  side  of  the  frsenum 
linguae.  Wharton's  duct — the  duct  of  the  submaxillary  gland — discharges 
itself  by  a  minute  opening  on  the  summit  of  the  barb. 

Structuee  of  the  Tongue.  The  tongue  possesses  a  mucous  covering, 
a  collection  of  mucous  glands,  a  median  fibrous  cord,  muscles,  nerves, 
and  connective-tissue. 

Mticous  Membrane  of  the  Tongue. — This  has  already  been  partly  noticed. 
Like  the  rest  of  the  lining  membrane  of  the  mouth,  it  has  a  stratified 
squamousepithelium.  It  is  intimately  adherent  to  the  subjacent  structures. 
It  is  thickest  on  the  dorsum  of  the  tongue,  where  it  is  harsh  and  wTinkled, 
On  the  sides  and  posterior  surface  of  the  tongue  it  is  thin  and  smooth. 


DISSECTION    OF    THE    HEAD    AND    NECK.  197 

The  Lingual  Fibrous  Cord. — This  is  a  fibrous  cord  extending  along 
the  middle  line  of  the  dorsum  of  the  tongue,  immediately  beneath  the 
mucous  membrane.  With  a  sharp  scalpel  incise  the  tongue  along  this 
line  to  the  depth  of  half  an  inch  or  more,  beginning  the  incision  be- 
hind the  circumvallate  papillae,  and  terminating  it  in  the  free  portion  of 
the  tongue.  This  will  expose  the  cord  in  its  entire  length.  It  is  a  little 
less  in  thickness  than  a  goose  quill.  It  begins  between  the  two  large 
circumvallate  papillae,  and  it  terminates  towards  the  junction  of  the  free 
and  the  fixed  portion  of  the  tongue,  being  about  seven  inches  in  length. 

Lingual  Glands. — These  are  aggregated  as  a  thick  layer  under  the 
mucous  membrane  at  the  upper  part  of  the  dorsum.  The  mucus  which 
they  secrete  coats  the  bolus  of  food  as  it  passes  through  the  fauces.  They 
are  of  the  racemose  type,  and  their  ducts  open  on  little  tubercles  of  the 
mucous  membrane. 

Directions. — The  mucous  membrane  is  to  be  raised  from  the  sublingual 
gland  and  the  sides  of  the  tongue.  It  is  convenient  to  describe  the 
gland  here,  although  it  is  not  a  part  of  the  tongue. 

The  Sublingual  Gland  (Plate  30).  This,  the  smallest  of  the 
salivary  glands,  is  placed  at  the  floor  of  the  mouth,  and  at  the  side  of, 
rather  than  under,  the  tongue.  It  extends  from  the  level  of  the  5th 
molar  tooth  to  the  symphysis.  It  is  in  contact  outwardly  with  the 
mylo-hyoid  mxiscle.  Inwardly  it  is  related  to  the  stylo-glossus,  genio- 
glossus,  and  genio-hyoideus  muscles,  to  Wharton's  duct,  and  to  the  lingual 
nerves.  Its  posterior  border  is  included  between  the  mylo-hyoid  and  the 
genio-hyoid  muscles,  and  is  related  inferiorly  to  the  submental  artery. 
Its  anterior  border  projects  the  mucous  membrane  at  the  side  of  the 
tongue,  so  as  to  form  the  sublingiial  ridge.  The  upper  extremity  is 
related  to  the  lingual  nerve  and  vein.  It  is  a  compound  racemose  gland, 
and  it  discharges  its  secretion  by  from  fifteen  to  twenty  ducts — the  dv^ts 
of  Rivinius — which  perforate  the  little  tubercles  on  the  sublingual  ridge. 

Directions. — The  gland  is  to  be  carefully  excised  without  injury  to 
Wharton's  duct  or  the  adjacent  vessels  and  nerves.  It  will  be  observed 
to  receive  a  nerve  from  the  lingual  branch  of  the  5  th,  while  its  vessels 
are  branches  of  the  submental  artery  and  vein. 

Wharton's  Duct  (Plates  30  and  31)  is  the  excretory  canal  of  the  sub- 
maxillary gland.  Leaving  the  lower  extremity  of  the  gland,  where  it  is 
crossed  outwardly  by  the  submaxillary  artery,  it  passes  between  the 
mylo-hyoideus  outwardly,  and  the  great  hyo-glossus  and  the  stylo-glossus 
muscles  inwardly,  its  position  here  being  immediately  behind  the  body 
of  the  hyoid.  A  little  in  advance  of  the  superior  extremity  of  the 
siiblingual  gland,  it  passes  to  its  inner  side,  and  is  continued  downwards 
between  the  gland  and  the  genio-glossus  muscle.  Finally,  it  opens  on 
the  summit  of  the  flattened  papilla,  or  harh,  at  the  side  of  the  fraenum 
linguae.     As  the  duct  passes  to  the  inner  side  of  the  sublingual  gland, 


198  THE  AX  ATOM  V  OF  THE  HOKSE. 

a  branch  of  the  lingual  nerve  turns  round  it.  The  duct  will  be  readily 
recognised  and  distinguished  from  a  blood-vessel  by  its  slender  and 
uniform  calibre,  and  by  its  clear  contents. 

The  Lingual  or  Gustatory  Nerve  is  a  branch  of  the  5th.  It  contains 
here,  however,  not  only  its  own  proper  fibres,  but  also  fibres  derived  from 
the  7th  through  the  chorda  tympani,  which  joins  it  in  the  first  part 
of  its  course.  At  the  root  of  the  tongue  it  passes  between  the  mylo- 
hyoideus  and  the  stylo-glossus  and  great  hyo-glossus  muscles,  imtil  it 
passes  to  the  inner  side  of  the  sublingual  gland.  There  it  turns 
forwards  between  the  muscles,  and  is  continued  in  a  fiexuous 
manner,  giving  off  branches  that  are  distributed  to  the  mucous  mem- 
brane of  the  tongue  in  its  lower  two-thirds.  At  the  base  of  the 
tongue  it  detaches  a  few  filaments  to  the  mucous  membrane  there,  to 
Wharton's  duct  and  the  submaxillary  gland,  and  a  larger  branch  for  the 
sublingual  gland  and  the  adjacent  mucous  membrane. 

The  Hypoglossal  (12th)  Nerve  (Plate  30)  will  be  found  at  the  base 
of  the  tongue,  in  front  of  Wharton's  duct,  where  it  is  included  between 
the  mylo-hyoid  and  great  hyo-glossus  muscles.  Passing  to  the  inner 
side  of  the  sublingual  gland,  it  comes  into  relation  with  the  lingual 
nerve,  and  divides.  The  branches  of  the  hypoglossal  are  motor  to  the 
muscles  of  the  tongue. 

The  Submental  Artery.  This  vessel,  a  branch  of  tlie  submaxillary, 
has  already  been  seen  in  the  dissection  of  the  intermaxillary  space.  It 
leaves  the  space  by  passing  forwards  through  the  mj-lo-liyoid  muscle, 
and  extends  along  tlie  posterior  border  of  the  sublingual  gland,  into 
which,  and  the  muscles,  it  throws  branches.  It  extends  beyond  the 
lower  extremity  of  the  gland,  and  terminates  in  small  branches  to  the 
mucous  membrane. 

The  Submental  Vein.  Tliis  is  relatively  larger  than  the  artery, 
which  it  accompanies. 

The  Lingual  Vein  (Plate  30).  This  vessel  will  be  found  at  the 
upper  part  of  the  tongue,  in  company  with  the  gustatory  nerve.  It 
receives  branches  from  the  soft  palate  and  pharynx,  and  joins  tlie  buccal 
A'cin. 

The  lingual  artery  (which  runs  its  course  se])arate  from  the  vein  of 
the  same  name)  and  the  9th  nerve  cannot  be  followed  mitil  some  of  the 
muscles  have  been  dissected.  Plate  31  will  serve  as  a  guide  in  the 
isolation  of  these  muscles. 

The  Mylo-hyoii)  Muscle  has  already  been  seen  in  the  dissection  of 
the  intermaxillary  space,  and  it  is  now  seen  on  its  opposite  aspect.  It 
is  described  at  page  171. 

The  Stylo-glossus.  {Hyo-glossus  lonyusoiVQYc\\A\\).  (Plate  31).  This 
is  a  long,  riband-shaped  muscle,  ansing  by  a  thin  aponeurotic  tendon 
from  the  outer  surface  of  the  great  coniu  of  the  hyoid  bone  near  its 


DISSECTION    OF    THE    HEAD    AND    NECK.  199 

lower  extremity.  It  extends  along  the  side  of  the  tongue  to  near  its 
tip,  where  its  fibres  are  confounded  on  the  middle  line  with  those  of  tlie 
opposite  muscle. 

Action. — To  retract  the  tongue,  and  at  the  same  time  to  incline  it 
laterally  if  only  one  muscle  acts. 

The  Great  Hyo-glossus  {Hyo-glosstis  hrevis  of  Percivall)  (Plate  31). 
To  expose  this  muscle  fully,  the  stylo-glossus  should  be  cut  near  its 
origin  and  reflected.  The  great  hyo-glossus  arises  from  the  lateral 
aspect  of  the  glossal  process,  body,  and  thyroid  cornu  of  the  hyoid  bone. 
Its  fibres  extend  obliquely  forwards  and  downwards  across  the  side  of 
the  fixed  portion  of  the  tongue,  and  turn  inwards  on  reaching  the 
dorsum. 

Action. — To  retract  and  depress  the  tongue. 

Directions. — Incise  the  origin  of  the  foregoing  muscle,  and  raise  it 
forwards  after  the  manner  of  Plate  32.  This  will  expose  more  fully  the 
other  muscles  of  tlie  tongue,  and  also  the  lingual  artery  and  the  9th 
nerve. 

The  Middle  Hyo-glossus  Muscle*  (Plate  32)  arises  from  the  front  of 
the  articulation  between  the  great  and  the  small  cornu  of  the  hyoid  bone. 
In  passing  downwards  beneath  the  great  hyo-glossus  it  crosses  over  the 
lingual  artery.  Its  fibres  are  confounded  in  front  with  those  of  the 
palato-glossus. 

Action. — It  is  a  feeble  retractor  of  the  tongue. 

The  Palato-glossus  (Plate  32).  This  is  a  small,  thin  muscle  ariswig 
from  tlie  edge  of  the  soft  palate.  It  passes  downwards  beneath  the 
great  hyo-glossus  and  over  the  lingvial  artery,  and  readies  the  root  of  the 
tongue. 

Action. — To  narrow  the  fauces. 

Directions. — The  next  two  muscles  lie  internal  to  tlie  small  cornu.  To 
expose  them,  the  middle  hyo-glossus  should  be  cut,  and  the  intercornual 
joint  pulled  outwards. 

The  Hyoideus  Transversus  is  peculiar  in  that  it  is  an  unpaired 
muscle  without  a  median  raphe.  It  extends  transversely  across  the 
middle  line,  being  attached  at  its  extremities  to  the  small  cornua  in  the 
whole  of  their  extent.  When  relaxed  it  passes  between  its  points  of 
attachment  with  a  curve  whose  concavity  is  directed  upwards  and 
forwards. 

Action. — To  raise  the  root  of  the  tongue. 

The  Small  Hyo-Glossus.  This  is  a  muscle  of  small  size  arisiiu/  from 
the  lower  extremity  of  the  small  cornu  and  from  the  body  of  the  hyoid 
bone.  It  passes  forwards  over  the  preceding  muscle  and  terminates  in 
the  root  of  the  tongue. 

*  This,  apparently,  is  the  muscle  first  described  by  Briihl,  in  1S50,  as  tiie  middle  dcsceadhig  sti/lo- 
filon^a.''. 


200  THE   ANATOMY    OF    THE    HOK.SE. 

Action. — To  aid  in  retracting  the  tongue. 

The  Genio-glossus,  or  Genio-hyo-glossus  (Plates  31  and  32).  This 
muscle  is  ftm-shaped.  Its  origin  is  from  a  depression  on  the  inner 
surface  of  the  horizontal  ramus  of  the  lower  jaw,  near  the  symphysis. 
From  this  point,  and  from  a  tendon  at  the  posterior  edge  of  the  muscle, 
its  fibres  radiate  into  the  tip,  centre,  and  base  of  the  tongue,  beneath 
those  of  the  stylo-glossus  and  great  hyo-glossus  muscles. 

Action. — The  upper  fibres  protrude  the  tongue  by  pulling  downwards 
its  base,  the  lower  fibres  retract  the  free  portion  of  the  tongiie,  and 
the  intermediate  fibres  (or  the  entire  muscle)  depress  the  tongue  as  a 
whole  towards  the  floor  of  the  mouth. 

If  the  dissector  will  raise  the  posterior  tendinous  edge  of  the  muscle, 
he  will  find  that  he  has  now  reached  the  middle  plane  of  the  tongue, 
which  is  here  occupied  by  a  quantity  of  connective-tissue  and  fat 
between  the  right  and  left  genio-glossus  nmscles. 

The  Genio-hyoideus  (Plates  31  and  32).  This  is  a  muscle  of  the 
hyoid  bone,  rather  than  of  the  tongue.  It  is  elongated  and  fusiform  in 
shape,  and  is  placed  beneath  the  tongue,  near  the  middle  line.  It  arises 
from  the  inner  surface  of  the  horizontal  ramus,  close  to  the  symphj'sis; 
and  passing  upwards  along  the  inner  edge  of  the  mylo-hyoid,  it 
becomes  inserted  into  the  glossal  or  spur  process  of  the  hyoid  bone. 

Action. — To  pull  forwards  the  hj-oid  bone. 

The  Lingual  Artery  (Plate  32).  This  is  a  large  branch  of  the  sub- 
maxillary artery,  and  has  already  been  seen  at  its  origin.  It  passes 
under  cover  of  the  great  hyo-glossus,  crossing  the  small  cornu  of  the 
hyoid  bone.  In  passing  to  the  root  of  the  tongue,  it  crosses  obliquely 
forwards  and  downwards  beneath  the  middle  hyo-glossiis  and  the  palato- 
glossus. In  the  body  of  the  tongue  it  lies  beneath  tlie  great  hyo-glossus, 
and  in  the  free  portion  of  the  tongue  it  is  internal  to  the  stylo-glossus. 
In  its  course  it  becomes  reduced  in  size  by  detaching  lateral  branches, 
and  it  terminates  at  the  tip  of  the  tongue  by  turning  inwards  and 
anastomosing  on  the  middle  line  with  the  vessel  of  the  opposite  side. 
Like  the  lingual  nerve,  the  artery  is  flexuous  in  the  inferior  part  t)f  the 
tongue,  that  it  may  be  adapted,  without  stretching,  to  the  varying 
length  of  the  organ. 

The  Glosso-pharyngeal  (9th)  Nerve  (Plate  32).  The  lingual 
continuation  of  this  nerve  will  be  found  crossing  the  inner  side  of  the 
articulation  between  the  great  and  small  cornua  of  the  hyoid  bone  to 
reach  the  base  of  the  tongue.  Here  it  divides  into  branches  for  the 
mucous  membrane  on  the  superior  third  of  the  tongue. 

THE   hard   palate   (fIG.    21). 

The  surface  of  the  hard  palate  is  covered  by  a  dense  mucous 
membrane  having  a  stratified  squamous  epithelium.     It  is  traversed  in 


DISSECTION    OP    THE    HEAD    AND    NECK. 


201 


its  entire  length  by  a  median  raphe,  and  is  crossed  from  side  to  side  by 
from  eighteen  to  twenty  curved  ridges.  The  concavity  of  the  ridges  is 
directed  upwards  (towards  the  root  of  the  tongvie).  The  ridges  are 
sharpest  and  the  interspaces  are  narrowest  at  the  upper  part  of  the 
palate.  A  layer  of  connective-tissue  with  numerous  veins  connects  the 
mucous  membrane  to  the  periosteum  of  the  bones  forming  the  basis  of 
the  palate. 


Fig.  21. 

Hard  Palate:  1.  Palato-labial  artery  of  right  side;  2.  Inosculation  of  riglit  and  left  arteries, 
forming  a  single  labial  artery  which  passes  forwards  through  incisor  foramen  ;  3.  Bar  of  carti- 
lage under  which  palato-labial  artery  runs. 


The  Palato-labial  Artery.  This  vessel  passes  along  the  side  of  the 
hard  palate,  resting  in  a  groove  on  the  bone,  close  to  the  alveoli.  An 
incision  should  be  made  down  to  the  artery  in  this  position,  and  it  should 
be  followed  backwards  and  forwards.  It  is  accompanied  by  a  satellite 
vein  and  nerve.  The  vessel  is  the  continuation  of  the  internal  maxillary 
artery.    Beginning  at  the  maxillary  hiatus,  it  reaches  the  upper  extremity 


202  THE  ANATOMY  OF  THE  HORSE. 

of  the  palate  by  traversing  the  palatine  canal.  It  descends  at  the  side 
of  the  palate,  and  at  the  level  of  the  corner  incisor  it  curves  in- 
wards towards  the  incisor  foramen,  where,  on  the  middle  line,  it  unites 
with  the  vessel  of  the  opposite  side.  The  single  lalnal  vessel  resulting 
from  this  union  passes  forwards  through  the  incisor  foramen  to  reach  the 
upper  lip.  In  passing  in  to  join  its  fellow  at  the  incisor  foramen,  the 
palato-labial  artery  runs  under  a  small  flexible  bar  of  cartilage,  which  is 
fixed  to  the  bone  by  its  upper  extremity,  while  its  lower  extremity  is 
free.  Whei-e  the  artery  curves  inwards,  it  detaches  a  branch  that 
passes  downwards  to  be  distributed  in  the  palate  below  the  level  of  the 
incisor  foramen. 

Palatine  Veins.  Over  the  whole  extent  of  the  hard  palate  there 
exists  a  rich  network  of  veins  in  the  submucous  connective-tissue.  This 
network  is  drained  by  a  large  vein  which  accompanies  the  palato-labial 
artery  as  far  as  the  lower  orifice  of  the  palatine  canal.  There  it  parts 
company  with  the  artery,  and  passes  along  the  staphyline  groove,  with 
the  staphyline  artery  and  nerve.  It  joins  the  alveolar  vein.  The 
variable  thickness  of  tlie  palate  depends  principally  on  the  amount  of 
blood  in  these  veins,  this  being  greatest  in  the  3'oung  animal. 

The  Palatine  Nerve  is  a  branch  of  the  superior  maxillary  division  of 
the  5th.  It  emerges  from  the  palatine  canal  along  with  the  artery, 
around  which  its  branches  interlace.  It  is  the  sensory  nerve  to  the 
hard  palate. 

THE    soft    palate,    Or    VELUM    PENDULUM    PALATI. 

This  is  an  oblique  valvular  curtain  placed  on  the  limit  of  the  oral  and 
pharyngeal  cavities.  The  oral  surface  of  the  curtain  looks  downwards 
and  backwards,  and  is  covered  by  miicous  membrane  continuous  with  that 
of  the  hard  palate.  The  pharyngeal  surface  has  the  opposite  direction, 
and  its  mucoxis  covering  is  continuous  with  that  of  the  nasal  chambers. 
The  anterior  edge  is  fixed  at  the  posterior  margin  of  the  hard  palate. 
The  lateral  edges  are  attached  on  the  limits  of  the  mouth  and  pharynx.. 
The  posterior  edge  is  free,  and  extends  across  the  root  of  the  tongue,  in 
front  of  the  epiglottis.  The  mucoiis  membrane  in  passing  between  the 
soft  palate  and  the  root  of  the  tongue  is  raised  on  each  side  into 
a  fold,  and  two  similar  but  less  prominent  ridges  of  mucoxis  membrane 
extend  fronx  the  soft  palate  to  the  sides  of  the  pharynx.  These  are 
termed  respectively  the  anterior  and  the  posterior  pillars  of  the  soft 
palate.  Between  them  is  a  space  which  marks  the  situation  of  the  tonsil 
in  most  mammals,  and  into  which  numerous  mucous  glands  open.  The 
soft  palate  of  the  horse  is  remarkable  for  its  large  size — a  fact  which 
explains  the  difficulty  with  which  the  horse  can  expire  or  eject  regurg- 
itated matters  through  the  mouth. 

The  Isthmus  of  the  Fauces.      This  is  the  aperture  of  communication 


DISSECTION    OF    THE    HEAD    AND    NECJlv.  203 

between  the  mouth  and  the  pharynx.  It  is  bounded  in  front  and  laterally 
by  the  free  edge  of  the  soft  palate,  and  by  its  anterior  pillars.  Behind 
it  is  bounded  by  the  extreme  upper  part  of  the  dorsum  of  the  tongue. 
In  the  horse,  owing  to  the  length  of  the  soft  palate,  this  aperture  is 
closed  except  during  the  passage  of  solids  or  liquids  in  deglutition. 

Structure  of  the  Soft  Palate.  This  comprises  two  layers  of  mucous 
membrane,  and,  included  between  these,  a  layer  of  mucous  glands,  a 
fibrous  aponeurosis,  muscles,  vessels,  and  nerves. 

Mucous  Alemhrane. — The  membrane  covering  the  oral  aspect  of  the 
curtain  is  directly  continuous  with  that  of  the  hard  palate,  and  through 
the  anterior  pillars  it  is  also  continuous  with  the  mucous  membrane  of 
the  tongue.  It  has  a  thick  tesselated  epithelium,  and  shows  numerous 
small  papillae  perforated  by  the  ducts  of  the  subjacent  glands.  The 
mucous  membrane  of  the  pharyngeal  surface  of  the  curtain  is  continuous 
with  that  of  the  nasal  chambers,  and  will  be  exposed  in  the  dissection  of 
the  pharynx. 

Stapliyline  *  Mucous  Glands. — These  form  a  thick  granular  layer  which 
will  be  exposed  by  removing  the  mucous  membrane  from  the  oral 
surface  of  the  soft  palate.  The  ducts  of  the  glands  open  on  the  oral 
surface  of  the  curtain,  and  the  bolus  of  food  th\is  gets  a  mucous  coating 
as  it  passes  through  the  isthmus. 

Fibrous  Aponetirosis. — If  the  glandular  layer  be  removed,  this  will  be 
exposed  in  the  anterior  part  of  the  curtain.  It  is  fixed  in  front  to  the 
margin  of  the  hard  palate ;  and  behind  it  is  continuous  with  the  palato- 
pharyngeus,  to  which  it  serves  as  a  tendon  of  origin. 

Muscles  of  the  Soft  Palate. 

The  Palato-glossus  Muscle  (Plate  32).  This  muscle  has  been  dis- 
sected with  the  tongue  (page  199). 

The  Palato  -  Pharvngeus  (Plate  32).  This  muscle  will  be  found 
beneath  the  glandular  layer  in  the  posterior  half  of  the  cui-tain.  At  the 
middle  line  it  is  continuous  with  the  opposite  muscle,  and  in  front  it  is 
continuous  with  the  fibrous  aponeurosis.  At  the  side  of  the  soft  palate 
it  is  continued  to  the  wall  of  the  pharynx ;  and  passing  beneath  the 
hyo-pharyngevis  muscle,  it  becomes  inserted  into  the  edge  of  the  thyroid 
cartilage.  In  the  latter  part  of  its  course  it  need  not  be  exposed  at 
present. 

Action. — To  tense  the  velum  and  carry  its  free  edge  upwards  towards 
the  pharynx. 

The  Tensor  Palati  (Plates  31  and  32).  This  muscle  and  the  next 
will  be  found  parallel  to  the  Eiistachian  tube,  and  on  its  outer  side.  It 
arises,  in  common  with  the  levator  palati,  from  the  styloid  process  of  the 
petrous  temporal  bone,  and  from  the  Eustachian  tube.      Its  terminal 

*  strictly  speaking,  tliis  adjective  applies  to  structures  pertaining  to  the  uvula,  but  it  may 
conveniently  be  used  to  distinguish  parts  belonging  to  the  soft  palate  of  the  horse,  in  wliicli  the  uvula 
is  not  developed. 


204  THE  ANATOMY  OF  THE  HORSE. 

tendon  is  reflected  inwards  on  the  havmlar  or  pnlley-like  process  of  tlie 
pterygoid  bone,  a  synovial  bnrsa  intervening,  and  expands  on  the  pos- 
terior surface  of  the  fibrous  aponeurosis  of  the  palate. 

Action. — To  tense  the  anterior  half  of  the  palate. 

The  Levator  Palati  (Plate  32).  This  muscle  is  placed  between  the 
preceding  and  the  Eustachian  tube.  It  arises  with  the  tensor  palati 
from  the  styloid  process  and  the  Eustachian  tube.  Reaching  the  upper 
wall  of  the  pharynx,  it  passes  beneath  the  pterygo-pharyngcus  to  gain 
the  pharyngeal  surface  of  the  palate,  where  it  expands. 

Action. — To  raise  the  velum  towards  the  roof  of  the  pharynx,  and 
thus  to  shut  off  the  communication  between  the  pharynx  and  the  nasal 
chambers. 

The  AzYGOs  UvuLiE,  which  in  man  is  situated  in  the  uvula,  was 
named  from  the  belief  that  it  was  a  single  muscle.  It  consists,  how- 
ever, of  right  and  left  halves  applied  together  on  the  middle  line  of  the 
soft  palate.  In  order  to  expose  them,  it  will  be  necessary  to  remove 
from  the  oral  surface  of  the  velum  a  portion  of  the  palato-pharyngeus 
muscles  on  each  side  of  the  middle  line.  The  origin  of  the  muscle 
is  from  the  fibrous  aponeurosis  of  the  palate,  and  it  terminates  at  the 
free  edge  of  the  curtain. 

Action. — To  raise  the  free  edge  of  the  velum. 

The  Staphylinb  Artery  is  a  slender  vessel  arising  from  the  internal 
maxillary  artery,  above  the  maxillary  hiatus.  It  reaches  the  velum  by 
passing  in  the  staphyline  groove,  along  with  the  nerve  of  the  same  name 
and  the  palatine  vein. 

The  Pharyngeal  Artery  (Plate  32),  a  branch  of  the  s\ibmaxillary, 
crosses  the  pharynx,  and  gives  its  terminal  branches  to  the  soft  palate. 

The  Veins  of  the  soft  palate  enter  either  the  lingual  vein  or  the 
palatine  vein,  which  lies  in  the  staphyline  groove. 

Nerves. — The  staphyline  nerve  will  be  found  with  the  artery,  in  the 
groove  of  the  same  name.  It  comes  from  the  superior  maxillary  division 
of  the  5th  nerve.  It  caiTies  not  only  scnsoiy  fibres  to  the  mucoiis 
membrane  of  the  palate,  but  also  motor  fibres,  which  come  from  the  7th 
nerve  through  the  spheno-palatine  ganglion,  and  are  distributed  to  the 
levator  and  probably  also  the  azygos  uvulpe. 

The  nerve  to  the  tensor  comes  from  the  5tli,  through  the  otic  gang- 
lion. 

THE  PHARYNX,  THE  HYOID  BONE,  AND  THE  BASE  OF  THE  SKULL. 

Directions. — Take  Plate  32  as  a  guide,  and  remove  the  greater  portion 
of  the  large  coniu  of  the  hyoid  bone,  making  the  upper  section  with  the 
bone-forceps  just  below  the  point  at  which  the  external  carotid  artery 
crosses  the  hinder  edge  of  the  cornu,  and  the  lower  a  little  above  the 
articulation  of  the  two  cornua.     Preserve  the  9th  nerve  at  the  posterior 


DISSECTION    OF    THE    HEAD    AND    NECK.  205 

edge  of  the  great  comu,  and  the  pharyngeal  artery  passing  beneath 
it. 

The  Palato-pharyngeus  has  already  been  in  part  dissected  with  the 
muscles  of  the  soft  palate.     See  page  203. 

The  Pterygo-pharyngeus.  This  muscle  arises  from  the  pterygoid  pro- 
cess, from  which  point  its  fibres  diverge  to  the  upper  and  lateral  aspect 
of  the  forepart  of  the  pharynx.  At  its  lower  edge  its  fibres  are  parallel 
to,  and  with  difficulty  separated  from,  those  of  the  palato-pharyngeus. 
Its  outer  surface  is  constantly  covered  by  a  layer  of  fatty  elastic  tissue. 

The  Hyo-pharyngeus,  migin — thjToid  or  heel  process  of  the  hyoid 
bone. 

The  Thyro-pharyngeus,  m'igin — thyroid  cartilage. 

The  Crico-pharyngeus,  origin — cricoid  cartilage. 

These  three  muscles  succeed  one  another  in  the  order  named,  the  first- 
mentioned  being  the  most  anterior.  They  pass  upwards  over  the  side  of 
the  pharynx,  and  terminate  on  the  middle  line  of  its  roof. 

The  Stylo-pharyngeus.  This  is  a  thin,  strap -like  muscle  arising 
from  the  inner  surface  of  the  great  cornvi  of  the  hyoid  bone  near  its 
upper  extremity.  It  descends  to  the  pharynx,  and  expands  on  it  at  the 
outer  edge  of  the  pterygo-pharyngeus. 

The  Small  Stylo-pharyngeus.  This  muscle  is  not  constant,  though 
freqviently  present.  It  is  a  delicate,  worm-like  muscle  arising  from  the 
inner  surface  of  the  great  comu,  about  an  inch  above  its  lower  extremity. 
It  passes  upwards  on  the  pharynx  at  the  anterior  edge  of  the  hyo- 
pharyngeus. 

The  Aryteno-pharyngeus.  This  muscle  will  not  be  seen  at  present. 
It  is  a  small  slip  arising  from  the  arytenoid  cartilage,  and  passing  to  the 
pharynx  at  its  junction  with  the  oesophagus. 

Action  of  the  Pharyngeal  Muscles. — The  stylo-pharyngeus  dilates  the 
anterior  part  of  the  pharynx  for  the  reception  of  the  bolus.  All  the 
other  muscles  are  constrictors,  grasping  in  succession  the  bolus,  and 
carrying  it  on  to  the  oesophagus. 

The  Cerato-hyoid  {Hyoideus  jmrvus  of  Percivall)  (Plate  33).  This, 
muscle  is  most  conveniently  dissected  at  this  stage,  and  it  is  therefore 
here  described  although  not  belonging  to  the  pharynx.  It  is  a 
small  flat  muscle  occupying  the  angle  between  the  small  and  the  thyroid 
cornu  of  the  hyoid  bone.  It  arises  from  the  posterior  edge  of  the 
small  cornu,  and  from  the  great  cornu  immediately  above  the  inter- 
comual  articulation.  It  is  inserted  into  the  upper  edge  of  the  thyroid 
cornu.  The  lingual  artery,  in  crossing  down  to  the  base  of  the  tongue, 
passes  over  the  muscle. 

Action. — To  elevate  the  thyroid  cornu  and  with  it  the  larynx. 

The  Pharyngeal  Artery.  This  is  a  small  branch  of  the  submaxillary 
which  passes  beneath  the   great   cornu,  crossing   either   outwardly  or 


206  THE  ANATOMY  OF  THE  HORSE. 

inwardly  the  9th  nci-vc.  It  gives  branches  to  the  pharynx,  and  is  con- 
tiniied  to  the  soft  palate. 

The  9th  or  Glosso-pharyxgeal  Nerve  descends  on  the  guttural 
pouch,  behind  the  great  comii ;  and  crossing  to  the  inner  side  of  the 
external  carotid  artery,  it  gives  off  its  pharyncfeal  branch.  This  turns 
round  behind  the  stylo-pharyngeus  muscle,  and  ramifies  on  the  pharynx, 
meeting  there  the  pharyngeal  branches  of  the  10th  and  sympathetic 
nerves,  and  forming  with  them  the  pharyngeal  jilexns. 

Nerve  to  the  Stylo-pharyngeus. — This  is  a  special  branch  of  the  glosso- 
pharyngeal, given  off  either  from  the  trunk  of  the  nerve  or  from  its 
glossal  continuation.  It  enters  the  muscle  at  the  middle  of  its  outer 
face. 

The  Glossal  Continuation  of  the  9th  nerve  passes  over  the  hyo-pha- 
ryngeus  and  palato-pharyngeus  to  reach  the  base  of  the  tongue ;  and  in 
its  course  it  detaches  fibres  which  are  probably  motor  to  the  palato- 
pharyngeus,  hyo-pharyngeus,  cerato-hyoid,  and  hyoideus  transversus.       . 

For  the  pharyngeal  branches  of  the  10th  and  sympathetic  nerves  see 
pages  194  and  195. 

Directions. — The  pharynx  is  to  be  opened  by  an  incision  along  its 
lateral  aspect,  Avhen,  by  hooking  back  the  edges  of  the  incision,  a  view 
of  its  interior  will  be  obtained. 

The  Pharyngeal  Cavity  (Fig.  24,  page  217)  is  inogularly  tubular  in 
form,  and  presents  seven  openings.     These  are  : — 

1.  The  Isthmus  of  the  Fauces,  already  described  (page  202). 

2.  The  Superior  Fares  (2),  situated  vertically  over  the  isthmus,  from 
which  they  are  separated  by  the  soft  palate.  They  are  wide,  gaping 
orifices,  with  rigid  bony  margins. 

3.  The  Lower  Openings  of  the  Eustachian  Tubes  (2),  which  are  sitiiated 
on  the  sides  of  the  pharynx,  behind  and  in  line  with  the  superior  nares. 
The  opening  has  the  form  of  a  vertical  slit,  the  outer  edge  of  which 
oontains  the  cartilaginous  extremity  of  the  tube. 

4.  The  Ujojier  Aperture  of  the  Larynx. — This  is  placed  on  the  floor  of 
the  pharynx,  and  has  the  form  of  a  pitcher-mouth.  During  deglutition 
the  epiglottis  folds  over  it  like  a  lid,  but  at  other  times  it  is  a  large 
patent  orifice. 

5.  The  (Esophageal  Orip'ce. — At  the  posterior  end  of  the  pharynx,  its 
cavity  is  continued  into  the  lumen  of  the  gullet.  Except  during  the 
passage  of  solids  or  fluids,  however,  this  orifice  is  not  open. 

Mucous  Membrane. — This  lines  the  cavity  completely,  and  is  con- 
tinuous through  the  before-mentioned  orifices  with  the  lining  membrane 
of  the  mouth,  nasal  chambers,  Eustachian  tubes  and  guttural  pouches, 
larynx,  and  oesophagus.  It  has  a  stratified  squamous  epithelium,  except 
in  its  upper  and  anterior  part,  where  it  is  ciliated.  It  has  many 
mucous  glands. 


DISSECTION    OF    THE    HEAD    AND    NECK.  J.V  I 

Directions, — Remove  the  larynx  by  cutting  across  the  remaining 
great  cornu  of  the  hyoid  bone,  and  the  root  of  the  tongue  in  front  of  the 
glossal  (spur)  process.  Cut  away  the  remains  of  the  pharyngeal 
muscles ;  and  after  examining  the  articulations  of  the  hyoid  bone,  set 
the  larynx  (and  the  attached  hyoid)  aside  for  futiire  examination.  (The 
larynx  is  described  at  page  224). 

Temporo-hyoidbal  Articulation.  This  joint  will  be  found  intact  at 
the  base  of  the  skull.  It  is  a  typical  amj^hiarthrodial  joint,  the  toe-like 
extremity  of  the  great  cornu  being  joined  to  the  hyoid  process  of  the 
petrous  temporal  bone  by  a  short  rod  of  cartilage  (about  half  an  inch  in 
length).  The  interposed  cartilage  is  sufficiently  long  and  flexible  to 
permit  a  considerable  range  of  movement.  By  these  joints  the  hyoid  is 
suspended  to  the  base  of  the  skull,  and  swings  backwards  and  forwards 
in  movements  comparable  to  Jiexion  and  extension.  These  movements 
are  concerned  in  the  actions  of  mastication  and  deglutition. 

The  Intercornual  Articulation.  This  is  another  amphiarthrodial 
joint,  the  opposed  extremities  of  the  great  and  the  small  cornu  being 
united  by  intermediate  cartilage.  In  this  cartilage  there  is  frequently 
to  be  found  a  small  pea-like  nucleus  of  bone  which  is  the  representative 
of  a  third  cornu — the  epihyal  of  comparative  anatomy. 

The  Basi-cornual  Articulation.  The  lower  extremity  of  the  small 
cornu  is  articulated  to  the  body  of  the  hyoid  in  a  small  diarthrodial 
joint  provided  with  a  capsular  ligament  and  a  synovicd  membrane.  The 
small  cornu  and  the  thyroid  cornu  form  at  this  joint  an  angle,  and  the 
movements  are  extension  and  Jiexion. 

Directions. — The  dissector  will  now  be  able  to  trace  the  anterior  root 
of  the  occipital  vein  to  its  origin  from  the  subsphenoidal  sinus,  and 
thereafter  the  anterior  and  lateral  straight  muscles  of  the  head  are  to 
be  dissected. 

The  Subsphenoidal  Sinus  (or  confluent).  This  is  a  venous  sinus 
placed  at  the  inner  edge  of  the  foramen  lacerum  basis  cranii.  It  extends 
downwards  (the  head  being  vertical)  for  half  an  inch  below  the  carotid 
notch,  the  sphenoid  bone  being  there  depressed  for  the  sinus.  At  this 
extremity  it  terminates  blindly.  Traced  upwards,  it  lies  along  the 
inner  edge  of  the  foramen  lacerum  (the  basilar  process  being  depressed 
beneath  it),  and  it  is  directly  continued  as  one  of  the  roots  of  the 
occipital  vein.  A  little  above  its  lower  extremity  the  sinus  is  pene- 
trated by  the  internal  carotid.  Slit  open  the  sinus,  and  observe  that 
it  communicates  by  an  oval  foramen  with  the  cavernous  sinus  of  the 
dura  mater.  This  oval  foramen  has  its  broad  end  circumscribed  by  the 
carotid  notch  of  the  sphenoid,  and  for  the  remainder  of  its  extent  it 
is  bounded  by  the  fibrous  tissue  that  fills  up  the  greater  part  of 
the  foramen  lacerum.  Through  the  forepart  of  this  oval  foramen 
the  carotid  artery  passes  from  the  subsphenoidal  to  the  cavernous  sinus, 


208  THE  ANATOMY  OF  THE  HORSE. 

the  artery  here  being  remarkable  in  that  it  is  actually  within  a 
vein. 

At  the  inferior  end  of  the  sinus  the  vidian  nerve  will  be  found  descend- 
ing in  the  vidian  groove  to  enter  the  canal  of  the  same  name  (page  214). 

The  Rectus  Capitis  Anticus  Major.  This  muscle  was,  for  the  most 
part  of  its  extent,  exposed  in  the  dissection  of  the  neck,  but  its  tendon 
of  insertion  remains  to  be  examined  now  (page  155). 

The  Rectus  Capitis  Anticus  Minor.  This  small  muscle  arises  from 
the  lower  aspect  of  the  ring  of  the  atlas  ;  and  passing  over  the  occipito- 
atlantal  joint,  at  the  side  of  the  preceding  muscle,  it  becomes  inserted 
into  the  basilar  process  of  the  occipital  and  the  body  of  the  sphenoid  at 
their  point  of  articulation. 

Action. — To  flex  the  occipito-atlautal  joint  (to  nod  the  head). 

The  Rectus  Capitis  Lateralis.  This  is  another  small  muscle  placed 
beneath  the  occipito-atlantal  joint.  It  arises  from  the  atlas,  external  to 
the  origin  of  the  preceding  muscle ;  and  passes  to  be  inserted  into  the 
styloid  process  of  the  occipital  bone. 

Action. — The  same  as  the  preceding  muscle. 

the  orbit. 

Directions. — With  the  saw,  cut  through  the  supraorbital  process 
external  to  the  foramen,  and  through  the  zygomatic  arch  at  either 
extremity.  The  palpebral  ligament  of  the  upper  lid  is  to  be  detached 
from  the  orbital  rim,  and  the  tri-radiate  piece  of  bone  marked  out  by 
these  sections  is  to  be  taken  away. 

The  orbital  cavity  in  the  skeleton  is  not  separated  from  the  temporal 
fossa,  but  in  the  living  animal  a  fibrous  membrane,  continuous  with  the 
periosteum  of  the  bones  circumscribing  the  cavity,  completes  the  orbit 
on  its  outer  side,  and  separates  it  from  the  temporal  fossa.  The  orbit 
is  thus  lined  and  completed  by  a  fibrous  membrane  of  a  conical  form, 
termed  the  ocular  sheath.  This  membrane  is  composed  of  fibrous  con- 
nective-tissue with  some  unstriped  muscular  fibre. 

Contents  of  the  Orbital  Cavity. — These  are :  the  eyeball  with  its 
muscles,  the  lachrymal  gland,  the  levator  palpebral,  the  membrana 
nictitans,  vessels,  nerves,  and  a  quantity  of  fat.  For  the  pi'oper  dissec- 
tion of  the  eyeball,  fresh  specimens  are  required,  and  it  is  therefore 
separately  described  at  page  257.  The  lachrymal  gland  and  the 
muscles  may  be  dissected  on  one  side,  the  other  being  reserved  for  the 
display  of  the  vessels  and  nerves. 

The  Lachrymal  Gland  is  the  organ  which  secretes  the  watery  fluid 
that  moistens  the  front  of  the  eye.  It  is  lodged  above  the  eye,  beneath 
the  supraorbital  process,  which  is  slightly  depressed  where  it  covers  the 
gland.  In  structure  it  is  of  the  racemose  type,  resembling  the  salivary 
glands,  but  being  looser  in  texture.     Its  secretion  is  discharged  by  a 


DISSECTION    OF   THE   HEAD   AND   NECK. 


209 


number  of  ducts  which  open  on  the  ocular  surface  of  the  upper  eyelid, 
close  to  the  temporal  canthus.     The  gland  is  to  be  removed. 

The  Levator  PALPEBRiE  Superioris  is  described  at  page  175. 

The  Membrana  Nictitans  and  its  connection  with  the  adipose  tissue 
of  the  cavity  are  described  at  page  173. 


Fig.  22. 

Muscles  of  the  Eyeball. 
1,  1.  Superior  oblique ;  2.  Fibrous  loop  for  the  same  ;  3.  Superior  rectus ;  4.  Internal  rectus ; 
5.  External  rectus ;  6.  Inferior  rectus  ;  7,  7,  7.  Fasciculi  of  the  retractor  ;  8.  Cut  origin  of  the  levator 
of  the  upper  eyelid  ;  9.  Nerve  to  inferior  oblique. 

Muscles  of  the  Eyeball. — These  are  seven  in  number,  viz.,  four  recti, 
one  retractor,  and  two  oblique.  (A  third  oblique  muscle  was  described 
by  the  late  Professor  Strangeways,  of  the  Dick  Veterinary  College,  but 
its  presence  is,  to  say  the  least,  not  constant.) 

Directions. — The  muscles  are  to  be  defined  by  detaching  the  eyelids 
and  conjunctiva  from  the  front  of  the  eye,  and  removing  the  levator 
palpebroe  and  the  loose  fat  which  forms  a  packing  material  between  the 
muscles.  Special  care  must  be  taken  not  to  injure  the  fibrous  arch  for 
the  tendon  of  the  superior  oblique  at  the  inner  side  of  the  orbit. 

The  Recti.  There  are  four  of  these,  distinguished  as  the  siq^erior 
rectus,  the  inferior  rectus,  the  external  rectus,  and  the  internal  rectus. 
They  ai-e  placed  one  above,  one  below,  and  one  on  either  side  of,  the  eye. 
They  have  all  a  flat  riband-like  form,  and  are  terminated  anteriorly  by 
aponeurotic  tendons.  They  all  take  origin  around  the  optic  foramen, 
and  each  becomes  inserted  into  the  forepart  of  the  sclerotic. 

The  Retractor  Oculi  is  placed  within  the  recti,  and  around   the 


210  THE  ANATOMY  OF  THE  HORSE. 

optic  nerve.     In  form  it  is  funnel-shaped,  forming  a  kind  of  sheath  to 
the  optic  nerve ;  but  frequently  it  is  divided  into  four  distinct  fasciculi, 


Fig.  23. 
Muscles  of  the  Eyeball. 

1.  Inferior  oUique  ;  2.  Inferior  rectus  ;  3.  External  rectus  ;  4.  Internal  rectus  ;  5.  Superior  rectus  ; 
0,  6.  Retractor  ;  7.  Nerve  to  inferior  oblique. 

one  lying  beneath  each  of  the  recti.  Its  origin  is  from  the  margin  of 
the  optic  foramen,  and  it  is  inserted  into  the  sclerotic. 

The  Superior  Oblique,  or  Trochlearis,  has  a  i-emarkable  disposition. 
It  arises  at  the  back  of  the  orbit,  and  passes  inwards  to  a  fibrous  arch, 
or  pulley,  through  which  it  plays  at  the  inner  wall  of  the  orbit,  below 
the  root  of  the  supraorbital  process.  Having  passed  through  this  arch, 
the  muscle  is  directed  outwards  above  the  eyeball ;  and  it  is  continued 
by  a  tendon  which  passes  beneath  the  superior  rectus  to  be  inserted  into 
the  sclerotic  on  its  outer  side,  between  the  insertions  of  the  superior 
and  external  recti. 

The  Inferior  Oblique.  This  muscle  has  its  origin  from  the  lachry- 
mal fossa  at  the  floor  of  the  orbit.  It  passes  outwards  below  the  eye- 
ball, and  becomes  inserted  into  the  sclerotic  between  the  insei-tions  of  the 
inferior  and  external  recti. 

Action  of  the  Muscles  of  the  Eyeball. — The  superior  and  the  inferior  rec- 
tus rotate  the  eye  around  a  horizontal  transverse  axis,  the  former  rolling 
it  upwards,  the  latter  downwards.  The  external  and  the  internal  rec- 
tus rotate  the  eye  around  a  vertical  axis,  the  first  rolling  it  outwards, 
the  second  inwards.      The  oblique  muscles  rotate  the  eye  around  an 


DISSECTION    OF   THE   HEAD   AND    NECK.  211 

antero-posterior  horizontal  axis,  the  superior  muscle  elevating  the  nasal 
angle  of  the  pupil,  while  the  inferior  muscle  depresses  it.  The  retractor 
oculi  pulls  the  eyeball  directly  backwards  into  its  cavity,  and  is  thus,  by 
pressure  exerted  on  the  orbital  fat,  instrumental  in  protruding  the  mem- 
brana  nictitans,  as  explained  at  page  173. 

The  Ophthalmic  Artery  is  a  branch  of  the  internal  maxillary,  from 
which  it  is  detached  within  the  subsphenoidal  canal.  Emei-ging  from 
that  canal  at  the  back  of  the  orbit,  it  is  directed  inwards  to  enter  the 
internal  orbital  foramen ;  and  in  this  course  it  passes  between  the 
superior  rectus  and  the  retractor  oculi,  where  the  latter  surrounds  the 
optic  nerve.  By  the  internal  orbital  foramen  it  reaches  the  forepart  of 
the  cranial  cavity.  It  is  further  described  at  page  238.  In  the 
orbital  part  of  its  course  it  gives  off  the  following  branches  : — 

1.  The  Sujiraorhital  Artery  ascends  on  the  inner  Avail  of  the  orbit  to 
pass  through  the  foramen  of  the  same  name. 

2.  The  Lachrymal  Artery,  distributed  to  the  gland  and  the  upper 
eyelid. 

3;  Muscular  Branches. 

4.  Ciliary  Branches  to  the  eyeball. 

5.  The  Central  Artery  of  the  Retina,  which  places  itself  in  the  axis  of 
the  optic  nerve,  and  enters  the  eyeball. 

The  Orbital  Branch  of  the  superior  dental  artery.  This  is  a  long 
and  slender  branch  detached  from  the  parent  vessel  before  it  enters  the 
superior  dental  canal.  It  creeps  over  the  floor  of  the  orbit  to  reach  the 
face,  where  it  anastomoses  with  the  submaxillary  artery  (Plate  29). 

Veins.  The  structures  within  the  orbit  are  drained  by  vessels  which 
unite  to  form  the  ophthalmic  vein.  This,  after  uniting  with  the  alveolar 
vein,  passes  into  the  cranial  cavity  by  the  foramen  lacerum  orbitale. 

The  Ophthalmic  Nerve  is  one  of  the  three  primary  divisions  of  the 
5th  nerve.  It  is  a  sensory  nerve,  and  divides  into  the  following  three 
branches,  which  issue  in  company  from  the  foramen  lacerum  orbitale  : — 

1.  The  Siipraorhital  Nerve,  accompanying  the  artery  of  the  same 
name. 

2.  The  Lachrymal  Nerve,  to  the  gland,  and  giving  off  a  branch  which 
traverses  the  ocular  sheath  to  reach  the  skin  over  the  temporal  fossa. 

3.  The  Palx)ehro-nasal  Nerve  divides  into  a  nasal  branch  which 
accompanies  the  ophthalmic  artery  through  the  internal  orbital  foramen, 
and  a  palpebral  branch  to  the  lower  eyelid  and  inner  canthus.  The 
palpebro-nasal  nerve  also  furnishes  the  sensory  filaments  to  the  ciliary 
ganglion,  through  which  it  supplies  sensory  fibres  to  the  eyeball. 

The  Orbital  Branch  of  the  superior  maxillary  division  of  the  5th 
nerve  issues  with  the  parent  nerve  from  the  foramen  rotundum.  It 
passes  at  the  outer  side  of  the  ocular  sheath  to  gain  the  temporal  can- 
thus  of  the  eyelids,  where  it  is  distributed.     It  is  sensory. 


212  THE  ANATOMY  OF  THE  HORSE. 

The  3rd  Cranial  Nerve,  or  Motor  Oculi,  issues  from  the  foramen 
lacerum  orbitale,  and  supplies  the  following  muscles : — the  superior, 
intenial,  and  inferior  recti ;  the  corresponding  fasciculi  of  the  retractor 
oculi ;  the  inferior  oblique ;  and  the  levator  palpebral  superioris.  It 
also  gives  the  motor  root  to  the  ciliary  ganglion,  and  thus  supplies  the 
ciliary  muscle  and  the  circular  fibres  of  the  iris. 

The  4th  Cranial  Nerve  (called  also  the  trochlear  or  j)athetic  nerve) 
issues  by  the  pathetic  foramen,  and  is  wholly  distributed  to  the 
superior  oblique  muscle. 

The  6th  Cranial  Nerve,  or  Abducens,  issues  by  the  foramen  lacerum 
orbitale,  and  is  distributed  to  the  external  rectus  and  the  subjacent 
fasciculus  of  the  retractor  oculi. 

The  Ciliary  Ganglion,  called  also  the  ophthalmic  or  lenticular 
ganglion.  This  minute  ganglion  should  be  sought  near  the  origin  of  the 
nerve  to  the  inferior  oblique  muscle.  Find  that  nerve  entering  the 
muscle,  and  trace  it  back  to  its  origin  from  the  motor  oculi.  From  its 
minute  size,  the  ganglion  is  likely  to  have  been  disturbed  in  the  previous 
dissection ;  and  in  order  to  display  it  satisfactorily,  a  special  dissection 
is  necessary.  The  nerves  which  pass  to  and  from  the  ganglion  may 
be  arranged  as  follows  : — 

Afferent  Branches. — (1)  A  motor  root  from  the  3rd  nerve,  (2)  a  sensory 
root  from  the  palpebro-nasal  nen-e,  (3)  a  sympathetic  root  from  the 
cavernous  plexus,  joining  the  ganglion  independently  or  (more  commonly) 
with  the  sensory  root. 

Efferent  Branches. — These  are  the  ciliary  nerves.  They  pierce  the 
sclerotic  and  are  distributed  to  the  eyeball,  and  will  be  again  refen-ed  to 
in  the  dissection  of  that  orgaia. 

The  optic  foi-amen,  through  which  the  optic  nerve  issues,  is  one  of  a 
group  of  foramina  termed  the  orbital  hiatus,  or  the  orbital  group  of 
foramina.  When  the  head  is  vertical,  there  lies  below  this,  at  the 
posterior  and  inner  part  of  the  orbit,  another  group  of  foramina — the 
maxillary  group  or  hiatus.  The  intenial  maxillary  artery  and  the 
superior  maxillary  division  of  the  5th  nerve  pass  between  these  two 
groups,  and  in  that  course  detach  several  important  branches.  Their 
dissection  is  conveniently  undertaken  after  that  of  the  muscles,  vessels, 
and  nerves  of  the  eye. 

The  Internal  Maxillary  Artery  issues  from  the  lower  orifice  of  the 
subsphenoidal  or  pterygoid  canal,  and  descends  to  the  maxillarj''  hiatus, 
where  it  is  directly  continued  as  the  palato-labial  arteiy.  While  within 
the  canal  it  detaches  the  ophthalmic  and  anterior  deep  temporal  arteries. 
After  its  emergence  it  gives  off  the  buccal,  superior  dental,  staphyline, 
and  spheno-palatine  arteries. 

The  Ophthalmic  Artery  issues  from  the  subsphenoidal  canal  along 
with  the  parent  artery.     It  has  already  been  followed. 


DISSECTION    OF    THE    HEAD    AND    NECK,  213 

The  Anterior  Deep  Temporal  Artery  passes  forwards  out  of  the 
subsphenoidal  canal  by  an  nn-named  foramen  above  the  edge  of  the 
orbital  hiatus.  It  is  expended  in  the  temporal  muscle  and  the  overlying 
skin. 

The  Buccal  Artery  is  detached  shortly  after  the  parent  vessel 
emerges  from  the  bone.  It  has  already  been  followed  in  its  distribution 
to  the  cheek. 

The  Superior  Dental  Artery  is  a  large  branch  which  enters  the 
superior  dental  canal.  The  vessel  is  continued  above  the  roots  of 
the  molar,  incisor,  and  canine  teeth.  Before  passing  into  the  canal,  it 
gives  off  an  orbital  branch  which  passes  across  the  floor  of  the  ox'bit  to 
reach  the  face.  Within  the  canal  it  emits  an  infraorbital  branch  which 
reaches  the  fixce  by  the  infraorbital  foramen. 

The  Staphyline  Artery  is  a  slender  branch  given  off  from  the 
posterior  aspect  of  the  internal  maxillary,  close  to  the  maxillary  hiatus. 
It  courses  along  the  staphyline  groove  to  reach  the  soft  palate. 

The  Sphbno-palatine  (Nasal)  Artery.  This  vessel  is  of  considerable 
size,  and  passes  at  once  through  the  spheno-palatine  foramen  to  be 
distributed  in  the  nasal  chamber. 

The  Palato-labial  Artery  is  the  continuation  of  the  internal  maxil- 
laiy.     It  passes  along  the  palatine  canal  to  reach  the  hard  palate. 

Veins.  At  this  point  the  veins  have  a  disposition  slightly  different 
from  the  arteries.  The  alveolar  vein — a  large  vessel  lying  on  the 
superior  maxilla  in  front  of  the  molar  teeth — turns  round  the  bone  and 
reaches  the  maxillary  hiatus.  Here  it  receives  superior  dental,  palatine, 
and  spheno-palatine  branches.  It  then  perforates  the  ocular  sheath, 
within  which  it  joins  the  opJitkalniic  vein.  The  opthalmic  vein  passes 
into  the  cranial  cavity  by  the  foramen  lace  rum  orbitale,  and  joins  the 
cavernous  sinus. 

The  superior  dental  and  spheno-palatine  veins  emerge  by  the  same 
foramina  as  the  corresponding  arteries.  The  palatine  vein,  however, 
does  not  issue  from  the  palatine  canal,  but  turns  round  the  bone  in  the 
staphyline  groove. 

The  Superior  Maxillary  Division  of  the  5th  Nerve.  This  sensory 
division  of  the  trifacial  emerges  from  the  cranium  by  the  foramen 
rotundum,  as  a  large  round  cord.  In  company  with  the  internal  maxil- 
lary artery,  it  descends  to  the  maxillary  hiatus,  where  it  enters  the 
superior  dental  canal.  Within  the  canal  it  gives  dental  branches  to  the 
roots  of  the  molar,  canine,  and  incisor  teeth,  and  then  issues  on  the  face 
at  the  infraorbital  foramen.  In  its  passage  between  the  orbital  and  the 
maxillaiy  hiatus,  it  gives  off  the  following  branches  : — 

1.  An  Orbital  Branch. — See  page  211. 

2.  The  Palatine  Nerve  accompanies  the  palato-labial  artery  into  the 
palatine  canal,  and  is  distributed  to  the  hard  palate. 


214  THE  ANATOMY  OK  THE  HORSE. 

3.  The  Staphyline  Nerve  passes  by  the  groove  of  the  same  name  to  the 
soft  palate. 

4.  The  Spheno-2X(latine  Nerve  enters  the  foramen  of  the  same  name, 
and  is  distributed  to  the  nasal  mucous  membrane. 

The  Spheno-palatine  (Meckel's)  Gangliox.  This  is  a  small,  greyish, 
elongated  and  fusiform  enlargement,  generally  adherent  to  the  spheno- 
palatine nerve.  Slender  branches  radiate  from  it,  and  are  divided  into 
afferent  and  efferent  filaments. 

Aff'erent  Filaments. — 1.  The  Vidian  Nerve,  which  enters  its  posterior 
extremity.  This  is  a  composite  nerve  foraaed  by  the  unimi  of  the  large 
superficial  petrosal  branch  of  the  7th  with  a  sympathetic  filament. 
Traced  upwards,  it  entei's  a  minute  foramen — the  lower  orifice  of  the 
vidian  canal.  At  the  upper  orifice  of  the  canal  it  enters  the  sub- 
sphenoidal  confluent,  and  passes  into  the  cavernous  sinus  by  the  foramen 
lacerum  basis  cranii.  There  it  separates  into  its  petrosal  and  sympa- 
thetic branches. 

2.  Short  bi-anches  passing  from  the  spheno-palatine  nerve  to  the 
posterior  part  of  the  ganglion. 

The  vidian  nerve  is  supposed  to  combine  the  motor  and  sympathetic 
roots  of  the  ganglion  ;  the  spheno-palatine  branches  represent  its  sensoiy 
root. 

Efferent  Branches. — Some  of  these  pass  to  the  ocular  sheath,  to  the 
ophthalmic  vessels,  and  to  the  muscles  and  other  accessory  pails  of  the 
eye.  Others  join  the  spheno-palatine,  palatine,  superior  dental,  and 
staph^'line  nerves.  The  latter  it  is  believed  derives  fi'om  this  source  the 
motor  filaments  which  it  conveys  to  the  levator  palati  muscle. 

THE    OCCIPITO-ATLANTAL    ARTICULATION. 

This  joint  possesses  two  synovial  sacs  and  an  enveloping  capsule,  with 
accessory  fasciculi  above  and  at  each  side  which  are  sometimes  dcsci'ibed 
as  distinct  ligaments — the  cruciform  and  styloid. 

The  Occipito-Atlantal  Ligament  is  membranous,  and  closes  the 
interval  between  the  occiput  and  atlas.  It  is  attached  to  the  occiput  at 
the  upper  and  lower  edges  of  the  foramen  magnum,  and  to  the  outer  side 
of  the  condyles.  Its  posterior  edge  is  fixed  to  the  anterior  border  of  the 
atlas.  The  most  superior  fibres  pass  obliquely,  the  right  and  left  fibres 
intercrossing.  This  is  the  so-called  cruciform  ligament.  On  each  side  a 
thickened  coixl-like  portion  passes  to  be  inserted  into  the  styloid  process 
of  the  occipital  bone,  and  these  constitute  the  styloid  ligaments. 

Synovial  Sacs.  Each  of  these  belongs  to  an  occipital  condyle  and  its 
receiving  cavity  on  the  front  of  the  atlas.  On  the  inner  side  each  is 
related  to  the  dura  mater  and  the  occipital  continuation  of  the  odontoid 
ligament,  and  elsewhere  tliey  are  supported  by  the  occipito-atlantal 
ligament. 


DISSECTION    OF    THE    HEAD    AND    NECK.  215 

Movements. — It  is  at  this  joint  that  the  nodding  movements  of  the  head 
are  executed. 

THE    ATLANTO-AXIAL    ARTICULATION. 

This  joint  possesses  four  ligaments  and  a  synovial  capsule. 

The  Inferior  Atlanto-axial  Ligament  is  riband-shaped,  and  stretches 
below  the  joint,  from  the  forepart  of  the  inferior  ridge  of  the  axis  to  the 
tubercle  of  the  atlas. 

The  Superior  Atlanto-axial  Ligament  is  exactly  like  the  interspinous 
ligament  of  the  succeeding  joints  of  the  neck.  It  consists  of  two  parallel 
bands  of  yellow  elastic  tissue  connecting  the  bones  above  the  joint. 

The  Atlanto-axial  Interannular  Ligament  is  membranous,  and 
connects  the  neural  arch  of  the  atlas  with  that  of  the  axis.  It  represents 
the  lig amentum  suhflavum  of  succeeding  joints. 

The  Odontoid  Ligament  is  placed  at  the  floor  of  the  spinal  canal  in 
this  region.  To  expose  it,  it  is  necessary  to  remove  the  upper  part  of 
the  ring  of  the  atlas.  It  is  strong,  flattened,  and  triangular.  It  is 
narrow  behind,  Avhere  it  is  fixed  to  the  depressed  upper  surftxce  of  the 
odontoid  process.  It  is  widest  in  front,  Avhere  it  is  fixed  to  the  floor  of 
the  atlas.  A  thin  continuation  of  the  ligament  is  carried  forwards  on 
each  side  to  be  attached  to  the  edge  of  the  foramen  magniun. 

Synovial  Sac.  This  is  supported  by  the  odontoid  ligament  above,  by 
the  inferior  atlanto-axial  ligament  below,  and  by  the  interannular 
ligament  laterally. 

Movements.  It  is  at  this  joint  that  the  movements  of  the  head  from 
side  to  side  are  executed.  In  these  movements  the  axis  remains  fixed, 
while  the  atlas  rotates  around  the  odontoid  process,  carrying  with  it  the 
head. 

temporo-maxillary  articulation. 

This  is  the  joint  formed  between  the  articular  surfjice  of  the  sqviamous 
temporal  and  the  condyle  of  the  inferior  maxilla.  An  interarticular 
fibro-cartilage  is  interposed  between  the  osseous  surfaces,  and  the  joint 
possesses  a  capsular  ligament  and  two  synovial  sacs. 

Capsular  Ligament.  This  envelopes  the  joint,  being  attached  around 
the  temporal  articular  surfoce  above,  and  around  the  condyle  of  the 
lower  jaw  below.  Its  inner  surface  is  adherent  to  the  interarticular 
cartilage.  In  front  and  inwardly  the  capsule  is  thin  and  membranous, 
but  behind  and  on  the  outer  side  it  shows  thickenings  which  are  some- 
times described  as  distinct  posterior  and  external  ligaments.  The  first  of 
these  stretches  from  the  post-glenoid  process  to  the  inferior  maxilla 
below  and  behind  the  condyle.  The  second  is  attached  above  to  the 
lower  edge  of  the  zygomatic  arch,  from  which  it  extends  downwards  and 
backwards  to  be  fixed  to  the  inferior  maxilla  below  and  external  to  the 
condyle. 


216  THE  ANATOMY  OF  THE  HORSE. 

The  Interarticular  Fibro-cartilage  should  be  exposed  by  removing 
the  capsuh\r  ligament  on  the  outer  side.  The  cartilage  extends  com- 
pletely across  the  joint,  which  it  divides  into  an  upper  and  a  lower  cavity. 
Its  upper  surfiice  is  a  cast  of  the  temporal  articular  surfoce,  while  its 
lower  is  moulded  on  the  condyle  of  the  jaw. 

Synovial  Sacs.  The  upper  of  these  belongs  to  the  ai-ticulation 
between  the  fibro-cartilage  and  the  temporal  articular  surface;  the  lower 
to  the  articulation  between  the  fibro-cartilage  and  the  condyle. 

Movements.  These  are  —  depression,  elevation,  2i'>'otraction,  retraction, 
and  lateral  movement  of  the  inferior  maxilla. 

When  the  jaw  is  depressed,  as  in  opening  the  mouth,  the  condyles  of 
both  jaws  are  can-ied  forwards,  taking  with  them  the  fibro-cax-tilages, 
until  they  lie  under  the  condyle  of  the  temporal  articular  surface  ;  and 
at  the  same  time,  the  maxillaiy  condyles  move  in  the  depression  on  the 
under  surface  of  the  interarticular  fibro-cartilage,  rotating  around  a 
transverse  axis.  When  the  lower  jaw  is  protracted,  the  movement 
consists  principally  in  antero-posterior  gliding  between  the  temporal 
articular  sui*face  and  the  interarticular  cartilage  ;  and  when  the  same 
movement  is  executed  alternately  on  opposite  sides,  a  lateral,  grinding 
action  is  produced. 

THE    cavity    of    THE    NOSE    (FIGS.   24  AND    25). 

Dii-ections. — Make,  with,  the  saw,  an  antero-posterior  vertical  section 
of  the  head,  a  little  to  one  or  other  side  of  the  mesial  plane,  taking  the 
mesial  stxtures  on  the  front  of  the  head  as  a  guide. 

The  cavity  of  the  nose  is  the  first  segment  of  the  air-passages,  and  is 
thus  a  part  of  the  respiratoiy  apparatus.  It  is  also  in  part  devoted  to 
the  sense  of  smell,  the  olftictory  nerve  being  distributed  over  a  part  of 
its  boundaiy  walls.  It  is  a  large  tubular  passage  tunnelled  through  the 
sk\ill  in  front  of  the  mouth  (the  head  being  vertical).  A  mesial  partition 
— the  septum  nasi — divides  the  passage  longitudinally  into  the  right 
and  left  nasal  fossce.  Each  nasal  fossa  may  be  described  as  having 
anterior,  posterior,  and  lateral  walls,  and  a  superior  and  an  inferior 
extremity. 

The  Anterior  Wall,  sometimes  termed  the  Hoof,  is  narrow  and  formed 
by  the  frontal  and  nasal  bones. 

The  Posterior  Wall,  sometimes  termed  the  Floor,  is  considerably  more 
extensive  than  the  roof  It  is  formed  by  the  palatine,  siipcrior  maxillary, 
and  premaxillary  bones,  b\it  in  much  greater  proportion  by  the  second 
of  these. 

The  Outer  Wall  is  formed  by  the  nasal  and  superior  maxillary  bones, 
and  is  occupied  by  the  anterior  and  posterior  turbinated  bones,  which 
project  into  the  cavity  and  separate  the  meatuses  of  the  nose  from  one 
Jtnother.     Thus,  the  anterior  meatus  is  the  narrow  interval  between  the 


DISSECTION    OP    THE   HEAD    AND   NECK. 


217 


anterior  (ethmoidal)  turbinated  bone  and  the  roof  of  the  cavity  (the 
nasal  bone) ;  the  middle  meatus  is  another  and  larger  interval  between 
the  two  turbinated  bones;  while  the  x>osUrior  meatus,  the  largest  of  these 


Fig.  24. 

Longitudinal  Section  of  the  Head,  showing  the  Cavities  of  the  Mouth,  Nose, 
AND  Pharynx  (Leyh). 

1.  Frontal  siniis ;  2.  Lateral  mass  of  ethmoid  bone  ;  3.  Anterior  meatus  of  nasal  chamber ;  4. 
Anterior  turbinated  bone  ;  5.  Middle  meatus  ;  6.  Posterior  turbinated  bone  ;  7.  Posterior  meatus  ; 
8.  Circumvallate  papillje  of  the  tongue  ;  9.  Section  of  soft  palate  ;  10.  Opening  of  right  Eustachian 
tube  on  side  of  pharynx ;  11.  Isthmus  of  the  fauces  ;  12.  Upper  aperture  of  the  larynx  ;  13.  Com- 
munication between  pharynx  and  cesophagus  ;  14.  Thyroid  body  ;  15.  Trachea. 

intervals,  is  included  between  the  posterior  (maxillary)  turbinated  bone 
and  the  floor  of  the  cavity. 

The  Inner  Wall  (the  septum  nasi)  is  partly  bony,  and  partly  cartila- 
ginous. In  its  upper  part  it  is  formed  by  the  bony  perpendicular  plate 
of  the  ethmoid,  and  at  its  posterior  edge  it  is  formed  by  the  vomer  bone  ; 
but  for  the  greater  part  of  its  extent,  the  partition  is  composed  of  a  plate 
of  cartilage — the  septal  cartilage.  This  septal  cartilage  is  continuous 
above  with  the  perpendicular  plate,  which  is  merely  an  ossified  portion 
of  it ;  behind  it  is  received  into  the  cleft  of  the  vomer,  and  expands  on 
the  premaxillaiy  suture ;  in  front  it  expands  on  the  inner  aspect  of  the 
iuternasal  suture ;  and  inferiorly  the  alar  cartilages  are  movably  con- 
nected to  it. 


218 


THE  ANATOMY  OF  THE  HORSE. 


The  Inferior  Extremities  of  the  nasal  fossae  are  termed  the  inferior  nares 
or,  in  common  hmguage,  the  nostrils.  They  have  already  been 
described  (page  176). 


Fig.  2.5. 
Transverse  Section  through  the  Nasal  Chambers. 
1.  Anterior  turbinated  bone;   2.   Posterior   turbinated  bone;    3.  Anterior  meatus;    4.   Middle 
meatus;  5.  Posterior  meatus ;  6.  Septum  nasi. 

The  Superior  Extremities  are  separated  from  the  cranial  cavity  by  the 
cribriform  plate  of  the  ethmoid  bone,  and  are  occnpied  by  the  lateral 
masses  of  the  same  bone.  Below  and  behind  these  are  the  superior 
nares — the  large  patent  orifices  by  which  the  nasal  fossae  communicate 
with  the  pharynx,  the  right  and  left  openings  being  separated  by  the 
vomer  bone. 

The  following  openings  into  the  nasal  fossa  should  be  found  : — 
1.  The  Opening  of  the  Lachrymal  Duct  {ductus  ad  nasum). — Look  for 
this  on  the  floor  of  the  nasal  fossa,  a  few  inches  within  the  nostril.  It 
is  easily  seen  in  the  living  animal,  and  has  already  been  referred  to  in 
connection  with  the  nostril  (page  176).  It  is  a  small  opening  (about  the 
same  diameter  as  a  goose  quill)  with  a  circular  outline,  having  an  appear- 
ance as  if  a  small  circle  of  skin  had  been  punched  out.  The  opening,  it 
is  to  be  observed,  is  on  the  skin,  and  not  the  mucous  membrane,  taking 
the  presence  of  hair  as  distinguishing  the  former  from  the  latter.  The 
duct  passes  upwards  beneath  the  mucous  lining  of  the  middle  meatus 
until  it  enters  the  osseous  tube  that  conducts  it  to  join  the  lachrymal 


DISSECTION    OF    THE    HEAD    AND    NECK.  219 

sac  at  the  floor  of  the  orbit.      The  lower  portion  of  the  tube  has  a  sti'ati- 
fied  epithelial  lining,  but  in  its  upper  part  the  epithelium  is  ciliated. 

2.  The  Opening  of  Stenson's  Canal. — Look  for  this  opening  on  the 
floor  of  the  nasal  fossa,  over  the  incisor  or  naso-palatine  cleft.  Pass  a 
flexible  probe  into  it.  It  will  be  found  to  pass  obliquely  into  the 
cartilaginous  substance  that  closes  this  opening.  It  there  joins  another 
canal — the  organ  of  Jacobson,  which  passes  upwards  at  the  side  of  the 
hinder  edge  of  the  septal  cartilage,  terminating  blindly  after  a  course  of 
four  or  five  inches.  The  organ  of  Jacobson  has  a  wall  of  hyaline  cartilage, 
with  a  mucous  lining,  and  numerous  mucous  or  serous  glands.  Its 
epithelial  lining  is  in  part  a  stratified  epithelium,  and  in  part  it  resembles 
the  olfixctory  epithelium  to  be  presently  described  ;  and  to  the  latter 
portion  some  fibres  of  the  olfactory  nerve  are  traceable. 

3.  The  Opening  of  Communication  with  the  Sinuses  of  the  Head. — This 
is  placed  towards  the  upper  extremity  of  the  middle  meatus.  Ordinarily 
it  has  the  form  of  a  curved  slit  not  visible  from  the  nasal  fossa ;  but  if  a 
flexible  probe  be  insinuated  between  the  two  turbinated  bones  at  this 
point,  it  may  be  guided  on  into  the  frontal  or  the  maxillary  sinus. 

The  Nasal  Mucous  Membrane  (Pituitary  or  Schneidercan  Membrane). 
As  already  seen  in  the  examination  of  the  nostrils,  the  skin  is  carried 
round  the  edges  of  these,  and  for  a  short  distance  into  the  nasal  fossa. 
Along  an  abrupt  line  it  loses  its  pigment  and  hair,  and  is  continued  by 
the  mucous  membrane.  This  mucous  membrane,  it  will  be  observed, 
differs  in  its  upper  and  its  lower  portions.  Thus,  in  its  lower  three- 
fourths  the  membrane  has  a  rosy,  vascular  tint,  while  in  its  upper 
fourth  it  is  distinguished  to  the  naked  eye  by  being  of  a  pale,  somewhat 
yellowish  colour.  The  first  of  these  may  be  termed  the  respirator?/ 
2)ortion  of  the  membrane,  as  distinguished  from  the  second,  or  olfactory 
portion.  The  former  has  a  stratified,  columnar,  ciliated  epithelium 
similar  to  that  of  the  air  passages  in  general,  and  in  its  submucous 
tissue  are  numerous  small  racemose  serous  or  mucous  glands.  The 
olfactory  mucous  membrane,  on  the  other  hand,  has  its  free  surface 
formed  by  a  layer  of  columnar  cells  for  the  most  part  non-ciliated ;  and 
between  the  bases  of  these  are  peculiar  spindle-shaped  olfactory  cells. 
The  olfactory  cells  are  connected  by  their  deep  ends  with  the  olfactory 
nerve  fibres,  while  their  opposite  extremities  are  insinuated  between  the 
columnar  cells,  and  terminate  on  the  surface  of  the  membrane  in  a 
few  stiff",  hair-like  processes.  In  the  submucous  tissue  are  numerous 
tubular  glands — the  glands  of  Boivman — which  open  on  the  free  surface 
of  the  membrane.  The  nasal  mucous  membrane  is  continuous  with  that 
of  Stenson's  canal  and  the  organ  of  Jacobson,  the  pharynx,  and  the 
sinuses  of  the  head. 

The  Olfactory  (1st  Cranial)  Nerve.  The  delicate  oval  swelling 
termed  the  olfactory  bulb,  whicli  is  lodged  in  the  fossa  of  the  same  name 


220  THE  ANATOMY  OF  THE  HORSE. 

at  the  forepart  of  the  cranial  cavity,  gives  off  from  its  suiface  the  olfac- 
tory nerve  fibres.  These  pass  in  bundles  through  the  foramina  of  the 
cribriform  plate  and  enter  the  nasal  fossa,  where  they  are  distributed  as 
a  network  in  the  olfactory  m\icous  membrane.  As  the  fibres  leave  the 
cranium,  they  cany  with  them  prolongations  from  the  membranes  of 
the  brain ;  and  they  are  remarkable  among  cerebro-spinal  nerves  in 
being  destitute  of  the  white  substance  of  Scwhann. 

Spheno-palatine  Nerve.  This  nerve,  already  seen  at  the  back  of  the 
orbit  as  a  branch  of  the  superior  maxillary  division  of  the  5th  (page  214), 
enters  the  nasal  fossa  by  the  spheno-palatine  foramen,  and  divides  into 
an  outer  and  an  inner  branch  for  the  nasal  mucous  membrane,  on 
which  it  confers  common  sensibility. 

The  Nasal  Branch  of  the  Ophthalmic  Nerve  (page  238)  is  another 
nerve  of  common  sensation.  Entering  the  upper  extremity  of  the  nasal 
fossa,  through  a  foramen  in  the  cribriform  plate,  it  ramifies  in  the 
mucous  membrane  on  both  sides  of  the  fossa. 

Vessels.  The  mucous  membrane  of  the  nasal  fossa  is  richly  supplied 
with  blood  by  the  spheno-palatine  artery  and  the  nasal  branch  of  the 
ophthalmic  arteiy  (pages  213  and  238),  satellites  of  the  two  preceding 
nerves.  The  veins  form  beneath  the  mucous  membrane  a  rich  plexus 
which  is  drained  principally  by  the  spheno-palatine  vein. 


DISSECTION    OF    THE    HEAD    AND    NECK. 


221 


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DISSECTION    OF    THE    HEAD    AND    NECK. 


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CHAPTER    V. 

DISSECTION  OF  THE  LARYNX. 

The  lar3'nx  is  a  short  tube  forming  the  upper  part  of  the  windpipe. 
It  is,  however,  not  merely  a  part  of  the  respiratory  apparatus,  but  is 
also  the  organ  of  voice.  It  possesses  a  framework  of  cartilages,  which 
are  movably  articulated  together,  and  connected  by  ligaments  or 
membranes.  These  cartilages  are  moved  by  muscles,  some  of  which 
pass  between  the  different  cartilages  and  constitute  an  intrinsic  group, 
while  others  pass  between  the  cartilages  and  extraneous  parts,  and  con- 
stitute an  extriTisic  group. 

Directions. — Provided  the  dissection  has  to  be  carried  out  on  one 
larynx,  the  study  of  the  muscles  must  precede  that  of  the  cartilages. 
When  another  larynx  can  be  procured,  it  is  more  advantageous  to 
reverse  this  order,  removing  the  muscles  from  the  fii-st  larynx  in  order  to 
study  the  cartilages  and  their  mode  of  union,  and  then  using  the  other 
for  the  examination  of  the  muscles  and  remaining  structures. 

Even  when  the  first  method  has  to  be  followed,  it  is  advisable,  before 
proceeding  to  dissect  the  muscles,  to  read  the  description  of  the  cartilages, 
which  is  therefore  here  put  first. 

CARTILAGES    OF    THE    LARYNX. 

These  are  five  in  number,  viz.,  the  cricoid,  thyroid,  and  epiglottis,  which 
are  single ;  and  the  pair  of  arytenoid  cartilages.  In  man  there  are  two 
additional  pairs — two  cornicula  laryngis  and  two  cuneiform  cartilages. 
In  the  horse  the  first  of  these  are  amalgamated  with  the  tips  of  the 
arytenoids,  while  the  cuneiform  cartilages  are  small,  shot-like  bodies 
included  in  the  aryteno-epiglottic  fold  of  mucous  membrane. 

In  the  natural  position  of  the  animal  at  rest,  the  long  axis  of  the 
larj^nx  is  oblique  upwards  and  forwards.  For  convenience  of  description, 
however,  we  may  assume  it  to  be  vertically  placed,  as  indeed  it  is  when 
the  head  and  neck  are  extended  (elevated)  to  the  fullest  degree.  In  this 
position  the  cartilages  are  related  to  one  another  as  follows  : — The  cricoid 
is  the  lowest,  and  is  connected  to  the  first  ring  of  the  trachea.  The  thyroid 
is  placed  above  this,  and  bounds  the  tube  of  the  larynx  in  front  and  at  the 
sides.  The  arytenoids  surmount  the  cricoid  behind,  and  the  epiglottis 
is  superposed  to  the  thyroid  in  front  of  the  upper  aperture  of  the  tube. 


DISSECTION    OP    THE    LARYNX.  225 

The  Cricoid  Cartilage  has  the  form  of  a  finger  ring,  from  which 
it  receives  its  name.  The  depth  of  the  ring  is  greatest  behind, 
where  it  presents  a  portion  comparable  to  the  bezel,  or  part  of  a  ring 
in  which  the  stone  is  set.  The  inner  surfece  of  the  ring  is  smooth, 
and  lined  by  the  laryngeal  mucons  membrane.  The  outer  surfixce  of 
the  bezel  is  divided  by  a  vertical  median  ridge  which  increases  the 
surface  of  origin  of  the  posterior  crico-arytenoid  muscle.  Towards  the 
outer  limit  of  this  surface  there  will  be  seen  on  each  side  a  little  cavity 
which  is  smooth  for  articulation  with  the  thyroid  cartilage.  The  inferior 
border  is  notched  in  the  middle  line  of  the  bezel,  and  is  connected  by 
ligament  to  the  first  ring  of  the  trachea.  The  upper  border  has  a  wide 
notch  in  front ;  and  posteriorly,  over  the  bezel,  it  shows  a  pair  of 
smooth  convex  facets  for  articulation  with  the  arytenoid  cartilages.  In 
texture  the  cricoid  is  composed  of  hyaline  cartilage. 

The  Thyroid  Cartilage  receives  its  name  from  covering  the  front  and 
sides  of  the  larynx  like  a  shield.  It  consists  of  a  median  thickened 
portion,  or  body ;  and  two  lateral  plates — the  alse,  or  wings.  The  body 
is  known  in  human  anatomy  as  "Adam's  apple" — the po7)m7)i  Aclami. 
The  epiglottis  is  superposed  to  it,  the  two  cartilages  being  united  by 
elastic  fibres.  On  each  side  it  is  continuous  with  the  wings.  Each  ala 
is  a  rhomboidal  plate  of  cartilage.  The  outer  surface  is  slightly  convex, 
and  the  inner  is  correspondingly  concave.  The  upper  edge  of  the  cartilage 
is  attached  to  the  thyroid  cornu  of  the  hyoid  bone  by  the  thyro-hyoid 
membrane.  The  lower  edge  slightly  overhangs  the  cricoid,  and  receives 
the  insertion  of  the  crico-thyroid  muscle.  The  other  two  edges  are 
directed  obliquely,  one  backwards  and  upwards,  the  other  forwards  and 
downwards.  The  first  of  these  receives  the  insertion  of  the  palato- 
pharyngeus  muscle ;  the  other,  in  receding  from  the  corresponding  edge 
of  the  opposite  wing,  leaves  beneath  the  body  a  triangular  gap  which  is 
occupied  by  the  crico-thyroid  membrane.  Of  the  four  angles  of  each 
plate,  three  demand  mention.  The  supero-anterior  angle  is  acute,  and 
joins  the  ala  to  the  body  of  the  thyroid.  The  supero-posterior  angle  is 
obtuse,  and  can-ies  a  small  bar  of  cai-tilage — the  superio}'  cornu  of  man — 
which  is  articulated  to  the  extremity  of  the  thyroid  cornu  of  the  hyoid 
bone.  Close  to  the  base  of  this  process  the  plate  is  perforated  by  a 
foramen  for  the  passage  of  the  superior  laryngeal  nerve.  The  postero- 
inferior  angle  is  acute  like  the  first,  to  which  it  is  diagonally  opposite. 
It  is  drawn  out  a  little,  forming  a  projection — the  inferior  cornu  of 
man — which  is  terminated  by  a  convex  facet  for  articulation  Avitli  the 
cricoid.     The  thyi-oid  is  composed  of  hyaline  cartilage. 

The  Arytenoid  Cartilages.  These  stand  at  the  upper  aperture  of 
the  larynx  like  the  mouth  of  a  pitcher,  and  from  this  resemblance  they 
are  named.  They  are  irregular  in  shape,  but  each  bears  some  resemblance 
to  a  three-sided  pyramid.     The  inner  surface  of  the  pyramid  is  covered 

Q 


226  THE  ANATOMY  OF  THE  HORSE. 

by  laryngeal  mucous  membrane  ;  the  outer  surfiice  receives  the  insertion 
of  the  thyro-arytenoid  muscle ;  the  posterior  surface  is  covered  by  the 
arytenoid  muscle.  The  base  of  the  cartilage  possesses  within  its  area  a 
smooth,  depressed  facet  for  articulation  with  the  cricoid.  Two  of  the  angles 
of  the  base  require  particular  notice,  viz.,  the  anterior  angle,  which  is 
pointed,  and  projects  horizontally  forwards  to  receive  the  insertion  of  the 
vocal  cord ;  and  the  postero-extemal  angle,  which  is  thick  and  rounded, 
and  receives  the  insertion  of  the  crico-arytenoid  muscles.  The  apex  is 
directed  upwards,  and  is  prolonged  by  a  slender  piece  of  yellow  fibro- 
cartilage  representing  the  cornicula  laryngis  of  man.  Tliis  curves  back- 
wards and  inwards ;  and  with  the  cori'esponding  process  of  the  opposite 
side  forms,  behind  the  upper  aperture  of  the  larynx,  the  pitcher- 
like lip.  Except  in  the  apical  prolongation,  which  is  composed  of 
yellow  or  elastic  fibi'o-cartilage,  the  texture  of  the  arytenoid  is  hyaline 
cartilage. 

The  Epiglottls  is  shaped  like  an  ovate,  pointed  leaf.  Its  anterior 
surface  is  concave  in  the  vertical  direction,  and  convex  from  side  to  side. 
Near  the  base  it  receives  on  the  middle  line  the  insertion  of  the  hyo- 
epiglottideus  muscle.  The  posterior  surface  has  the  converse  configura- 
tion, and  presents  numerous  pits  in  which  are  lodged  mucous  glands. 
The  borders  of  the  cai-tilage  are  convex,  and  they  are  free  above,  but 
below  they  are  enveloped  by  the  aryteno-epiglottic  folds  of  mucous  mem- 
brane. The  apex  is  pointed,  and  curved  forwards  in  the  iipright 
position  of  the  cartilage.  The  base  of  the  cartilage  is  expanded,  and 
rests  on  the  body  of  the  thyroid.  Fi-om  each  side  of  it  an  irregular  bar 
of  caililage  projects  horizontally  backwards.  The  epiglottis  is  composed 
of  yellow  fibro-cartilage. 

ARTICULATIONS,    LIGAMENTS,    AND    MEMBRANES    OF    THE    LARYNX. 

Mode  of  Union  with  the  Hyoid  Bone.  The  larynx  is  suspended  to  the 
base  of  the  skull  through  the  intervention  of  the  hyoid  bone,  the  tip  of 
the  thjToid  comu  (heel  process)  of  that  bone  being  connected  by  liga- 
mentous fibres  (without  a  synovial  membrane)  to  the  so-called  siqyeHor 
comu  at  the  supero-posterior  angle  of  the  thyroid  ala.  The  connection 
between  the  hyoid  bone  and  the  larynx  is  furtlier  maintained  by  the 
thyro-hyoid  membrane,  which  is  attached,  on  the  one  hand,  to  the  body  and 
thyroid  cornua  of  the  hyoid,  and,  on  the  other,  to  the  body  and  upper 
edge  of  each  wing  of  the  thyroid  cartilage. 

Mode  of  Union  with  the  Trachea.  The  lower  edge  of  the  cricoid 
cartilage  is  connected  to  the  first  ring  of  the  trachea  by  a  fibro-elastic 
membrane — the  aico-tracheal  lu/ament. 

Union  of  the  Cricoid  and  Thyroid  Cartilages.  The  postero-inferior 
angle,  or  inferior  cornu,  of  each  thyroid  ala  is  articulated  to  the  concave 
facet  on  the  bezel  jof  the  cricoid  in  a  diarthrodial  joint,  provided  with  a 


DISSECTION    OF    THE    LARYNX.  227 

small  capsular  ligament,  and  lined  by  a  synovial  sac.  The  two  cartilages 
are  further  united  by  the  crico-thyroid  viemhrane.  This  is  a  fibro-elastic 
structure  consisting  of  a  central  and  two  lateral  portions.  The  central 
portion  is  triangular  and  fills  up  the  space  between  the  adjacent  edges 
of  the  right  and  left  thyroid  alse.  It  is  attached  by  its  sides  to  these 
edges,  while  by  its  base  it  is  inserted  into  the  upper  boi-der  of  the 
cricoid.  Each  lateral  portion  lies  imder  cover  of  the  laryngeal  mucous 
membrane,  and  is  shaped  somewhat  like  a  quadrant,  having  an  inferior 
convex  edge  fixed  to  the  margin  of  the  cricoid  in  company  with  the  central 
portion,  an  anterior  edge  confounded  with  the  central  portion,  and  an 
upper  straight  edge  which  is  thin  and  free  on  the  side  of  the  larynx. 
This  upper  edge  is  the  true  vocal  cord ;  and  since  its  fibres  are  attached 
in  front  to  the  angle  of  union  of  the  thyroid  alse,  and  posteriorly  to  the 
projecting  anterior  angle  of  the  base  of  the  arytenoid,  it  is  also  termed 
the  thyro-arytenoid  ligament.  Vocal  sounds  are  produced  by  the  vibra- 
tion of  the  vocal  cords. 

Movements. — The  movements  between  the  cricoid  and  thyroid  cartil- 
ages take  place  around  an  imaginary  horizontal  axis  passing  through  the 
right  and  left  crico-thyroid  joints,  and  in  these  movements  either  cartil- 
age may  be  supposed  to  remain  fixed  while  the  other  revolves  around 
the  axis.  It  should  be  observed  that  these  movements  vary  the  distance 
between  the  angle  of  jiinction  of  the  thyroid  alse  and  the  base  of  the 
arytenoids,  and  thus  vary  the  tension  of  the  true  vocal  cords,  which 
stretch  between  these  points. 

Union  of  the  Cricoid  and  Arytenoid  Cartilages,  Each  arytenoid 
cartilage  is  articulated  by  the  concave  facet  on  its  base  to  one  of  the 
convex  fixcets  on  the  upper  edge  of  the  ci'icoid  bezel.  It  is  a  diarthrodial 
joint,  possessing  a  capsular  ligament  and  a  synovial  sac. 

Movements. — The  arytenoid  cartilage  swings  like  a  door,  around  a 
vertical  axis  passing  through  the  crico-arytenoid  joint.  When  the 
cartilage  is  swung  outwards,  the  true  vocal  cord,  which  is  attached  to 
the  anterior  angle  of  its  base,  is  separated  from  the  cord  of  the  opposite 
side,  and  the  glottis  is  widened.  The  glottis  is  narrowed  by  the  opposite 
movement. 

Union  of  the  Thyroid  and  Epiglottis.  These  cartilages  are  united 
by  elastic  fibres  passing  between  tliem,  and  forming  a  kind  of  amphi- 
arthrosis. 

Movements. — Except  during  the  act  of  deglutition,  the  epiglottis  stands 
erect  in  front  of  the  upper  aperture  of  the  larynx.  During  that  act 
the  cartilage  is  bent  downwards  and  backwards  so  as  to  cover  the 
aperture  like  a  lid.  This  movement,  however,  is  executed  not  exclusively 
at  the  joint  between  the  two  cartilages,  but  partly  by  a  bending  of  the 
whole  cartilage.  At  the  close  of  the  act  of  deglutition  the  epiglottis 
assumes  the  erect  position,  owing  to  its  own  elastic  texture  and  the 


228 


THE  ANATOMY  OF  THE  HORSE. 


elastic  fibres  connecting  it  to  the  thyroid ;  but  in  the  horse  this  action 
is  assisted  by  the  hyo-epiglottideus  muscle. 


THE  MUSCLES  OF  THE  LARYNX. 

Extrinsic  Group. — This  includes  the  sterno-thyroid,  the  th}TO-hyoid, 
and  the  hyo-epiglottideus.  The  last  of  these  is  a  single  muscle  ;  the 
other  two  are  double. 

The  Sterno-thyroid  Muscle  (Fig.  26).     See  page  146. 
The  Thyro-hyoid  Muscle  (Figs.  26  and  27).    This  is  a  dark-coloured, 
fleshy  muscle  taking  orir/in  from  the  thyroid  comu  (heel  process)  of  the 
hyoid  bone,  and  inserted  into  an  oblique  line  on  the  outer  surface  of  the 
thyi'oid  wing. 

Action. — Acting  alone,  the  thyro-hyoid  muscles  would  elevate  the 
larynx  between  the  thyroid  cornua  of  the  hyoid  bone  ;  biit  when  they 
act  in  concert  with  the  sterno-thyroid,  the  thyroid  cartilage  will  be 
steadied,  and  will  serve  as  the  fixed  point  for  the  crico-thyroid  and  thyro- 
arytenoid muscles. 

The    Hyo-epiglottideus    Muscle   (Fig.   27)   takes   origin  from   the 

upper  face  of  the  body  of  the 
h^'oid  bone ;  and  passing  back- 
wards in  the  middle  line,  it  is 
inserted  into  the  anterior  sxirface 
of  the  epiglottis  at  its  lower  part. 
Its  fibres  are  mixed  with  a  quan- 
tity of  flitty-elastic  tissue. 

Action. — To  assist  the  natxiral 
elasticity  of  the  epiglottis  in 
restoring  the  cartilage  to  the 
erect  position  at  the  close  of  the 
act  of  deglutition. 

Intrinsic  Grou}). — This  includes 
four   pairs  of  muscles,   viz.,  the 
crico-thyroid,  the  thyro-aiytenoid, 
the  posterior  crico-arytenoid,  and 
the  lateral  crico-arytenoid;    and 
a  single  muscle — the  arytenoid- 
eus. 
The  Crico-thyroid   Muscle  (Fig.    26)  arises  from  the  side   of  the 
cricoid  cartilage  ;  and  its  fibres,  passing  obliquely  upwards  and  back- 
wards, are  inserted  into  the  lower  edge  of  the  thyroid  wing. 

Action. — This  muscle  acts  on  the  crico-thyroid  joint,  increasing  tlic 
tension  of  the  vocal  cord  by  increasing  the  distance  between  the  fore- 
part of  the  thyroid  and  the  base  of  the  arytenoid  cartilage.     In  this 


Fig.  2G. 
Larynx,  Side  View. 
1.  Glossal  Process  of  Hyoid;  2.  Small  Cornu ; 
3.  Great  Comu  ;  4.  Arytenoid  Cartilage  ;  5.  Thyro- 
Hyoideus;  6.  Insertion  of  Sterno -Thyroid  ;  7.  Crico- 
Thyroideus  ;  8.  Crico-arytenoideus  Posticus ;  9.  Ist 
Ring  of  Trachea  ;  10.  Thyroid  Body. 


DISSECTION    OP    THE    LARYNX. 


229 


action  either  the  cricoid  or  thyroid  attachment  may  be  the  fixed  point 
of  the  fibres. 

Directions. — The  thyro-arytenoid  and  lateral  crico-arytenoid  muscles 
lie  under  cover  of  the  thyroid  wing,  which  must  therefore  be  removed 
on  one  side.  This  is  to  be  done  by  removing  the  thyro-hyoid  and  crico- 
thyroid muscles,  disarticulating  the  cri co-thyroid  joint,  and  incising  the 
ala  a  little  behind  the  body  of  the  thyroid,  after  the  manner  of  Fig.  27. 

The  Thyro-arytbnoid  Muscle  (Fig.  27)  consists  of  two  parallel 
bundles,  between  which  the  mucous  inembrane  of  the  ventricle  of  the 
larynx  protrudes  as  a  pouch.  Its  fibres  arise  from  the  inner  surface  of 
the  thyroid  wing  near  its  junction  with  the  body,  and  from  the  crico- 
thyroid membrane.  The  lower  fibres  are  {■^serted  into  the  outer  surface 
of  the  arytenoid  cartilage,  while  its  higher  fibres  join  those  of  the 
arytenoideus  muscle. 

Action. — The  muscle  is  antagonistic  to  the  crico-thyroid,  dimin- 
ishing the  tension  of  the  vocal  cord  by  acting  on  the  crico-thyroid 
joint. 

The  Posterior  Crico-ary- 
tenoid Muscle  (Figs.  26  and 
28).  This  is  the  most  powerful 
of  the  intrinsic  muscles.  Its 
muscular  tissue  is  dark  red, 
and  mixed  with  tendinous 
tissue.  Its  fibres  take  origin 
from  the  outer  surface  of  the 
cricoid  bezel,  and  are  inserted 
into  the  prominent  tubercle 
on  the  external  angle  of  the 
arytenoid  cai'tilage. 

Action. — To  swing  out- 
wards the  arytenoid  carti- 
lage, and  thus  to  separate 
the  vocal  coi-ds  and  dilate 
the  glottis. 

The  Lateral  Crico-ary- 
tenoid Muscle  (Fig.  27). 
This  muscle  is  placed  below 
the  thyro-arytenoid,  under 
concealment  of  the  thyroid 
wing.  Its  fibres  arise  from  the  upper  border  of  the  side  of  the  cricoid 
cartilage ;  and  passing  backwards  and  upwards,  they  become  inserted 
into  the  same  tubercle  on  the  base  of  the  arytenoid  as  the  posterior 
muscle,  and  into  the  outer  surface  of  the  arytenoid  in  front  of  that 
tubercle. 


Larynx,  Side  View  (thyroid  ala  iikmoved). 

1.  Glossal  Process  of  Hyoid ;  2.  Cut  Base  of  Thyroid 
Cornu  ;  3.  Small  Cornu  ;  4.  Great  Cornu  ;  5.  Epiglottis  ; 
6.  Arytenoid  Cartilage  ;  7.  Cut  Wing  of  Thyroid  Carti- 
lage ;  8.  Facet  on  Cricoid  for  Articulation  with  Thyroid 
Cartilage  ;  0.  Pouch  of  Mucous  Membrane  from  Ven- 
tricle of  Larynx  ;  10.  and  11.  Upper  and  Lower  Bundles 
of  Thyro -Arytenoideus  ;  12.  Crico-Arytenoideus  Lateralis; 
13.  Crico-Arytenoideus  Posticus ;  14.  Thyro-Hyoideus ; 
15.  Hyo-Epiglottideus  ;  IG.  Thyroid  Body  ;  17.  1st  Ring 
of  Trachea. 


230 


THE  ANATOMY  OF  THE  HORSE. 


Action. — The  muscle  acts  on  the  crico-arytenoid  joint  in  a  manner 
antagonistic  to  the  preceding  muscle,  approximating  the  vocal  cords  and 
uaiTOwing  the  glottis  by  swinging  the  arytenoid  cartilage  inwards. 

The  Arytenoideus  Muscle  (Fig.  28). 
This  may  be  regarded  either  as  a  single 
muscle,  or  as  a  double  muscle  whose  right 
and  left  fibres  meet  at  a  median  raphe.  Its 
fibi'es  are  inserted  on  each  side  into  the  pos- 
terior surface  of  the  ar^-tenoid  cartilage, 
and  superiorly  it  is  joined  by  the  higher 
fibres  of  the  thyro-arytenoid  muscle. 

Action. — To  approximate  the  right  and 
left  arytenoid  cartilages,  and  thus  narrow 
the  glottis. 

nerves  of  the  larynx. 
Two  nerves  are  distributed  to  the 
larynx — the  superior  and  inferior  laryn- 
geal nerves.  The  latter  is  also  known 
as  the  recurrent  nerve,  and  both  are 
branches  of  the  vagiis,  or  10th  cranial 
nerve. 

The  Superior  Laryngeal  Nerve  has  its 
origin  described  at  page  194,  It  gives 
motor  filaments  to  the  crico-thyroid  and 
crico-pharyngeus  muscles ;  and  then  pene- 
trating the  thyroid  wing  by  the  foramen 
near  its  supero-posterior  angle,  the  nerve 
splits  into  sensory  branches  distributed  to  the  mucous  membrane  of  the 
larynx,  giving  also  twigs  to  the  lining  of  the  pharynx  and  oesophagus. 

The  Inferior  Laryngeal  (Recurrent)  Nerve  has  its  origin  and 
course  described  at  page  149.  It  is  the  motor  nerve  to  all  the  intrinsic 
muscles  except  the  crico-thyroid,  and  it  also  gives  some  sensory  twigs  to 
the  laryngeal  mucous  membrane. 


Fig.  28. 
Lakynx,  Back  View. 
1.  Epiglottis;  2.  Arytenoid  Carti- 
lage ;  3.  Thyroid  Cartilage ;  4.  Ary- 
tenoideus  ;  5.  Crico-Arytenoideus  Pos- 
ticus ;  6.  Cricoid  Cartilage ;  7.  1st 
Ring  of  Trachea ;  8.  Thyroid  Body. 


INTERIOR   OF    THE    LARYNX. 

Directions. — A  vertical  incision  should  be  made  along  the  middle  line 
of  the  larynx  behind,  severing  the  arytenoideus  muscle  and  the  bezel  of 
the  cricoid  cartilage.  By  separating  the  lips  of  this  incision,  a  view  of 
the  interior  of  the  larynx  from  behind  will  be  obtained,  and  tliis  is  to 
be  supplemented  by  looking  into  the  tube  from  its  upper  and  lower 
apertures. 

The  Superior  Aperture  of  the  larynx  is  a  large  orifice  placed  at  the 
floor  of  the  phaiynx.     It  is  bounded  in  front  by  the  epiglottis,  behind 


DISSECTION    OF    THE    LABYNX. 


231 


by  the  pitcher-like  lip  of  the  arytenoid  cartilages  and  the  fold  of  mucous 
membrane  uniting  them,  and  laterally  by  the  aryteno-epiglottic  fold  of 
mucous  membrane.     During  degluti- 
tion the  epiglottis  is  folded  over  the 
aperture,  which  it  closes  like  a  lid. 

The  Lower  Aperture  is  circum- 
scribed by  the  inferior  edge  of  tlie 
cricoid  cartilage,  and  is  directly  con- 
tinued by  the  lumen  of  tlie  trachea. 

The  Glottis,  or  Rima  Glottidis. 
This  is  a  third  aperture,  placed  about 
the  middle  of  the  tube  of  the  larynx, 
which  it  divides  into  an  upper  and  a 
lower  compartment.  In  its  anterior 
two-tliirds  this  opening  lies  between 
the  light  and  left  vocal  cords,  and 
in  its  posterior  third  it  lies  between 
the  bases  of  the  arytenoid  cartilages. 
The  size  of  the  aperture  is  varied  by 
the  movements  executed  in  the  crico- 
arytenoid joints,  as  ah-eady  seen  ;  and 
its  form  varies  with  its  size.  It  can 
be  completely  closed  by  the  apposition 
of   its  margins    in  the  mesial  plane.  Pic,  29. 

When  it  is  only  slightly  opened,  it  is  i,.,er.or  of  the  Larvnx,  seen  from  behind 

a    slit-like      anterO-pOSterior     aperture^        i.  Epiglottis ;   2.   Arytenoia   cartilage ;   21. 
•i,,.i  .  1  ]„j.„l        Its  base  where  it  bounds  the  glottis;  3.  Cut 

Widest  at  the  centre  ;  when  nK)derately  ^.^rtace  of  cricoid  cartilage  ;  1  Vocai  cord; 
open,  as  in  easy  respiration,  it  has  the  ^i Jr*"''" °^ *^'  larynx;  o.  Sub-epigiottic 
form  of  an  elongated  isosceles  triangle 

with  the  base  behind  ;  when  dilated  to  the  fullest  extent,  it  is  lozenge- 
shaped. 

The  Ventricles,  or  Sinuses,  of  the  larynx.  Each  of  these  is  a  recess, 
or  cavity,  placed  on  the  side  of  the  larynx.  The  entrance  to  it  lies  above 
the  vocal  cord,  whose  free  straight  edge,  covered  by  mucous  membrane, 
forms  the  lower  margin.  The  upper  margin  is  formed  by  a  concave  fold 
of  mucous  membrane,  containing  in  man  a  few  fibres  designated  the 
false  vocal  cord.  The  cavity  of  the  ventricle  descends  to  the  outer  side 
of  the  true  vocal  cord,  and  a  pouch  of  the  mucous  lining  of  the  cavity 
passes  out  between  the  upper  and  lower  divisions  of  the  thyro-arytenoid 
muscle. 

The  Sub-epiglottic  Sinus  is  a  depression  beneath  the  base  of  the 
epiglottis,  and  provided  with  a  lunated  fold  of  mucous  membrane. 

The  Sub- arytenoid  Sinus  is  a  depression  beneath  the  crico-arytenoid 
joints. 


232  THE  ANATOMY  OP  THE  HORSE. 

Mucous  Membrane  of  the  Larynx.  This,  which  is  coutiimuus  with 
the  Uning  of  the  pharynx  and  trachea,  is  of  a  pale  colour.  It  forms 
the  aryteno-cpiglottic  folds,  and  lines  the  ventricle  of  the  larj-nx.  It  is 
provided  with  nximcrous  mucous  glands.  Its  free  suiface  is  covered  by 
an  epithelium,  which  is  ciliated  except  over  the  vocal  cords  and  around 
the  superior  aperture,  in  which  positions  it  is  stratified  and  squamous. 


CHAPTER   VI. 

DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON. 

DirectUMs. — The  removal  of  the  brain  of  the  horse  from  its  contain- 
ing cavity  is  a  somewhat  difficult  operation,  in  consequence  of  the 
thickness  of  the  cranial  bones.  Supposing  the  head  of  an  animal 
recently  killed  to  have  been  procured  for  the  special  purpose,  the  first 
steps  are  the  disarticulation  of  the  jaw  on  both  sides,  and  the  removal 
of  the  inferior  maxilla.  Next  denude  the  cranial  bones  of  the  muscles 
and  other  soft  structures,  and  with  the  saw  remove  on  each  side  the 
zygomatic  arch,  the  supraorbital  process  of  the  frontal,  and  the  styloid 
process  of  the  occipital.  Estimating  the  thickness  of  the  last-named 
bone  at  the  poll,  as  much  as  possible  of  it  may  be  sawn  off  without 
actually  encroaching  on  the  cranial  cavity.  Armed  with  a  chisel, 
mallet,  and  strong  bone-forceps,  the  student  must  now  remove  as 
much  of  the  cranial  wall  as  will  enable  him  to  extract  the  brain  ; 
and  he  may  do  this  by  removing  either  the  roof  or  the  floor  of  the 
cavity.  The  first  method  is  the  speedier,  but  the  latter  has  the 
advantage  of  permitting  the  roots  of  the  cranial  nerves,  the  pituitary 
body,  and  the  cranial  vessels  to  be  better  preserved.  The  dura  mater  is 
to  be  left  as  far  as  possible  intact,  but  its  attachments  along  the  inter- 
frontal  and  interparietal  sutures,  and  to  the  oblique  ridge  between 
the  cerebral  and  cerebellar  divisions  of  the  cranial  cavity,  must  be  cut 
with  the  scalpel.  When  the  forepart  of  the  cavity  is  reached,  the 
handle  of  the  scalpel  is  to  be  used  to  scoop  the  olfactory  bulbs  out  of 
the  fossae  in  which  they  lie. 

The  brain  having  been  removed  in  its  membranes,  it  should  be  laid 
with  its  base  upwards  on  a  broad  strip  of  calico,  and  lowered  into  a 
vessel  of  methylated  spirit  or  a  ten  per  cent,  solution  of  nitric  acid  in 
water.     After  a  week's  immersion,  it  will  be  ready  for  examination. 

MEMBRAXES,    OR    MENINGES,    OF    THE    BRAIN. 

The  brain,  like  the  spinal  cord,  is  surrounded  by  thi-ee  envelopes  : 
the  dura  mater,  the  arachnoid,  and  the  pia  mater. 

The  Dura  Mater  is  the  external  of  these  envelopes.  It  is  a  strong 
fibrous  membrane,  similar  in  structure  to  the  spinal  dura  mater,  wdth 
which  it  is  continuous  at  the  foramen  magnum.  It  differs,  however, 
from  the  same  envelope  of  the  spinal  cord,  in  that  it  is  closely  adherent 


234  THE   ANATOMY    OF   THE   IIORSK 

to  the  inner  surface  of  the  cranial  bones,  and  forms  for  them  an  intenaal 
periosteum.  All  over  its  outer  surface  it  is  connected  by  slender  fibrous 
processes  and  vessels  to  the  bones ;  but  it  is  particularly  adherent  to 
these  along  the  lines  of  the  sutures,  and  at  the  margins  of  foramina. 
The  meningeal  vessels  ramify  on  the  outer  surface  of  the  membrane, 
and  leave  their  impressions  on  the  inner  surface  of  the  cranial  bones. 
Sometimes  the  outer  surface  of  the  dura  mater,  on  each  side  of  the 
middle  line  above,  shows  numbers  of  granular  processes — the  Pacchion- 
ian bodies,  which  are  developed  from  the  subjacent  arachnoid.  Occa- 
sionally they  are  large  enough  to  cause  the  partial  absorption  of  the 
bones  over  them.  The  inner  surface  of  the  dura  mater  is  smooth,  in 
virtue  of  an  endothelial  layer  representing  the  parietal  layer  of  the 
arachnoid.  This  inner  surfoce  is  closely  applied  to  the  brain  contained 
within  the  other  two  membranes  ;  and  along  certain  lines  it  detaches 
processes  which  pass  inwards,  and  form  partial  partitions  between  the 
different  divisions  of  the  cncephalon.  These  processes  are :  the  falx 
cerebri  and  the  tentorium  cerebelli. 

The  Falx  Cerebri  is  a  vertical,  mesial,  sickle-shaped  process  which  dips 
in  between  the  two  hemispheres  of  the  cerebrum.  The  convex  upper 
edge  of  the  process  is  attached  to  the  cristagalli  process,  and  to  the  inter- 
frontal  and  interparietal  sutures.  The  concave  lower  edge  is  thin  and 
lace-like,  and  rests  free  on  the  corpus  callosum.  The  short  posterior 
edge,  or  base,  is  straight,  and  is  attached  to  the  intracranial  projection 
of  the  interparietal  bone. 

The  Tentorium  Cerebelli  is  a  vaulted  partition  extending  transvei'sely 
between  the  cerebrum  and  the  cerebellum.  In  outline  it  is  crcscentic, 
having  a  superior  convex,  and  an  inferior  concave,  border.  The  former 
is  attached  on  the  middle  line  to  the  intracranial  projection  of  the  inter- 
parietal bone,  and  on  each  side  of  that  its  attachment  descends  obliquely 
forwards  and  downwai'ds  along  the  crest  formed  by  the  parietal  and 
petrous  temporal  bones.  The  concave  edge  is  free,  and  arches  over 
the  cnxra  cerebri.  The  anterior  surface  of  the  membrane  is  convex, 
and  the  posterior  ends  of  the  cerebral  hemispheres  rest  on  it.  The 
posterior  surface  is  concave,  and  is  in  contact  with  the  cerebellum. 

The  Sinuses  of  the  Dura  Mater. — These  are  venous  passages  formed 
by  the  splitting  of  the  dura  mater.     They  arc  as  follows  : — 

The  Superior  Longitudinal  Sinus  is  of  considerable  size,  and  is 
found  in  the  falx  cerebri  at  its  attached  or  convex  edge.  Beginning  at 
the  crista  galli  process,  it  becomes  larger  as  it  passes  backwards,  and  it 
terminates  at  the  intracranial  projection  of  the  interparietal  bone. 

The  Inferior  Longitudinal  Sinus  is  small  and  inconstant.  It 
extends  along  the  free  or  concave  edge  of  the  falx  ;  and  after  receiving 
the  veins  of  Galen,  it  is  continued  backwards  in  the  tentorium  cerebelli, 
terminating  at  the  same  ])oint  as  tlie  preceding  sinus. 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  235 

Where  the  two  foregomg  sinuses  meet,  they  form  the  whirlpool  of 
Herophilus  {torcular  Herophili),  from  which  the  blood  is  drained  away 
by  the  transverse  sinuses. 

The  Transverse  Sinuses  pass  right  and  left  at  the  periphery  of  the 
tentorium  cerebelli,  and  enter  the  parietotemporal  conduit.  In  that 
canal  each  is  continued  as  the  parieto-temporal  confluent,  from  which 
the  blood  is  drained  away  by  the  roots  of  the  temporal  veins. 

The  Cavernous  Sinuses.  Each  of  these  is  placed  in  the  dura  mater 
at  the  side  of  the  sella  turcica  of  the  sphenoid  bone.  Anteriorly  each 
receives  the  ophthalmic  vein,  and  posteriorly  the  right  and  left  sinuses 
become  continuous  behind  the  pituitary  gland.  The  venous  arch  which 
they  thus  form  discharges  its  blood  through  the  foramen  lacerum  basis 
cranii  into  the  sub-sphenoidal  confluent.  The  internal  carotid  artery 
traverses  the  cavernous  sinus,  and  forms  while  in  it  a  sigmoid  curve. 

The  Petrosal  Sinuses  are  small,  and  pass  in  the  tentorium  cerebelli 
on  each  side,  between  the  transverse  and  cavernous  sinuses. 

The  Occipital  Sinuses.  These  are  placed  in  or  external  to  the  dura 
mater  lining  the  cerebellar  division  of  the  cranial  cavity.  They  are 
continuous  through  the  foramen  magnum  with  the  spinal  simises,  and 
their  contained  blood  is  drained  away  by  a  large  vein  that  passes  through 
the  condyloid  foramen  to  join  the  occipital  vein. 

The  Meningeal  Arteries.  These  are  derived  from  the  meningeal 
branch  of  the  ophthalmic  artery,  Avhich  enters  the  forepart  of  the  cavity 
at  the  internal  orbital  foramen ;  and  from  the  great  meningeal  or 
sphe7io-spinous  branch  of  the  internal  maxillary.  The  sjyheno-spinous 
artery  enters  by  the  foramen  lacerum  basis  cranii,  and,  after  detaching 
meningeal  branches,  enters  the  pai'ieto-temporal  conduit  to  anastomose 
with  the  mastoid  artery.  Some  meningeal  twigs  are  also  furnished  by 
the  prevertebral  branch  of  the  occipital  artery  (page  191). 

The  Meningeal  Nerves.  Filaments  from  the  4th,  5th,  9th,  and 
10th  cranial  nerves,  and  from  the  sympathetic,  are  said  to  have  been 
traced  to  the  dura  mater. 

The  Arachnoid.  This,  like  the  same  membrane  of  the  spinal  cord,  is 
a  delicate  transparent  membrane.  In  structure  and  disposition  it  is 
comparable  to  a  serous  membrane.  Its  2)arietal  layer  is  represented  by 
the  endothelial  lining  of  the  dura  mater ;  its  visceral  layer  invests  the 
brain  and  pia  mater;  and  the  parietal  and  visceral  portions  together 
enclose  a  space,  which  is  the  arachnoid  cavity,  or  subdural  space.  The 
free  surface  of  the  membrane  bounding  this  space  is  smooth  and  moist 
like  a  sei-ous  membrane.  Between  the  visceral  arachnoid  and  the  pia 
mater  another  space  is  left,  which  is  termed  the  subarachnoid  space.  This 
space  is  most  evident  over  the  intervals  between  the  cerebral  convolu- 
tions, and  over  surface  depressions  at  the  base  of  the  brain,  for  at  these 
points  the  arachnoid  does  not  dip  down  to  line  the  hollows,  but  bridges 


236 


TUB    ANATOMY    OF    THK    HORSE. 


them  over.     The  space  is  continuous  with  the  same  space  in  the  spinal 
meninges,  and  contains  the  limpid  cerebrospinal  fnid. 

The  PiA  Mater.  This  is  the  vascular  membrane  of  the  brain.  It 
consists  of  delicate  areolar  tissue  and  bloodvessels.  It  invests  the 
brain  closely,  following  all  its  surface  irregularities.  Behind  the  cerebral 
hemispheres  it  sends  towards  the  interior  of  the  cerebrum  a  Avide  process 
— the  velum  into }x>situm  ;  and  where  the  cerebellum  is  superposed  to  the 
medulla  oblongata,  it  forms  on  each  side  a  thickened  granular  cord — the 
choroid  plexus  of  the  J^th  venti-icle.     These  will  be  exposed  at  a  later  stage. 

ARTERIES    OF   THE    BRAIN    (fIG.    30). 

Three  vessels  are  conceraed  in  supplying  blood  to  the  encephalou, 
viz.,  the  basilar,  internal  carotid,  and  ophthalmic  arteries. 


Fii; 

The  Aktekies  ok  the  Bkain. 
1.  Anterior  branch  of  ceiebro-spinal  artery  ;  2.  Basilar  artery  ;  3,  3.  Irregular  branches  to  medulla 
and  cerebellum  ;  4.  Posterior  cerebellar  arteries ;  5.  Bifurcation  of  the  basilar ;  0.  Anterior  cere- 
bellar arteries ;  7.  Posterior  cerebral  arteries  (more  numerous  and  sni.aller  than  usual);  8.  Internal 
carotid ;_  9.  Posterior  communicating  branch  ;  10.  Anterior  branch  of  internal  carotid,  which  divides 
to  form  11  and  12— the  middle  and  anterior  cerebral  arteries  ;  13.  Single  vessel  formed  by  the  union 
of  11  and  12,  disappearing  into  great  longitudinal  fissure  ;  A.  Medulla  oblongata  ;  B.  Pons  Varolii ; 
C.  Cerebellum  ;  U.  Crus  cerebri ;  E.  Corpus  albicans ;  F.  Optic  commissure  ;  G.  Olfactory  bulb  ; 
H.  Cerebral  hemisphere. 

The  Basilar  Artery  is  formed  on  the  middle  line  of  the  lower  face  of 
the  medulla  oblongata,  by  the  union  of  two  vessels.     These  are  the 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  237 

anterior  divisions  of  the  right  and  left  cerebro-spinal  arteries,  whose 
posterior  divisions  nnite  in  the  same  manner  to  form  the  middle  spinal 
artery.  The  basilar  artery  passes  forwards  in  the  median  groove  of  the 
medulla,  and  crosses  the  pons,  in  front  of  which  it  bifurcates  to  form 
the  posterior  cerebral  arteries.  In  its  course  the  basilar  artery  detaches 
on  each  side,  besides  numerous  vessels  to  the  medulla  and  pons,  the 
posterior  cerebellar  arteries. 

The  Posterior  Cerebellar  Arteries  are  two  in  number,  a  right  and  left. 
They  are  detached  at  different  levels  from  the  basilar,  behind  the  pons ; 
and  they  turn  round  the  medulla  to  reach  the  cerebellum. 

The  Posterior  Cerebral  Arteries  diverge  from  each  other  in  the  inter- 
peduncular space ;  and  after  being  connected  together  by  a  short 
transverse  branch  of  considerable  volume,  and  by  numerous  smaller 
reticulate  twigs,  they  are  joined  by  the  posterior  communicating  branch 
of  the  internal  carotid.  Each  then  turns  outwards  over  the  crus 
cerebri  to  gain  the  choroid  plexus  and  the  posterior  part  of  the  cerebral 
hemisphere.  Behind  the  point  at  which  the  vessels  are  comiected  by 
the  short  transverse  branch,  they  give  off"  the  anterior  cerebellar  arteries. 
Sometimes,  as  in  Fig.  30,  the  posterior  cerebral  artery,  instead  of  turnino- 
outwards  as  a  single  vessel,  detaches  from  its  outer  side  two  or  three 
branches  which  wind  round  the  crus. 

The  Anterior  Cerebellar  Arteries  are  variable  in  number  and  disposi- 
tion, and  may  arise  as  branches  of  the  basilar  artery.  Generally  there 
are  two  or  three  on  each  side,  and  they  turn  backwards  and  outwards 
over  the  crus  cerebri  to  gain  the  front  of  the  cerebellum. 

The  Internal  Carotid  Artery.  This  vessel  begins  above  the  cricoid 
cai-tilage  of  the  larynx,  as  one  of  the  terminal  branches  of  the  common 
carotid.  It  passes  upwards  and  forwards  to  the  foramen  lacerum  basis 
cranii,  being  sustained  in  a  fold  of  the  guttural  pouch,  and  accompanied 
by  some  nervous  branches  from  the  superior  cervical  ganglion  of  the 
sympathetic.  Piercing  the  sub-sphenoidal  sinus,  it  passes  through  the 
foramen  into  the  cavernous  sinus,  Avithin  which  it  forms  a  sigmoid  cui-ve. 
It  then  leaves  the  sinus,  and  gaining  the  deep  face  of  the  dura  mater, 
it  divides  at  the  margin  of  the  sella  turcica  of  the  sphenoid  bone  into 
an  anterior  and  a  posterior  branch.  The  latter,  termed  the  posterior 
communicating  artery,  is  reflected  backwards  to  join  the  posterior  cerebral 
artery.  The  anterior  branch  passes  forwards,  and  at  the  outer  side  of 
the  optic  commissure  divides  into  the  middle  and  anterior  cerebral 
arteries. 

The  Middle  Cerebral  Artery  passes  outwards  across  the  hemisphere,  in 
the  fissure  of  Sylvius. 

The  Anterior  Cerebral  Artery  unites  in  the  mesial  plane,  above  the 
optic  commissure,  with  the  corresponding  vessel  of  the  opposite  side. 
The  single   vessel  thus  formed  receives  the  meningeal  branch  of  the 


238  THE  ANATOMY  OF  THE  HORSE. 

ophthalmic  artery,  and  turns  round  the  anterior  end  of  the  corpus  callo- 
sum  to  gain  the  great  longitudinal  fissure.  Here  it  separates  into  a 
right  and  a  left  branch,  each  of  which  passes  backwards  along  the  flat 
face  of  the  hemisphere. 

By  the  anastomosis  of  the  two  anterior  cerebral  arteries  in  front,  and 
the  junction  of  the  posterior  communicating  artery  on  each  side  with 
the  posterior  cerebral,  which  results  from  the  bifurcation  of  the  basilar 
artery,  a  vascular  circle  is  established  around  the  pituitar}'  body.  This 
is  termed  the  Circle  of  Willis,  and  its  object  is  to  keep  up  a  free  blood 
supply  to  the  cerebmm,  even  should  there  be  an  obstruction  in  one  of  the 
main  vessels  forming  the  circle.  Moreover,  the  internal  carotid  ai-teries 
of  opposite  sides  are,  before  they  divide,  connected  by  a  large  transvei'se 
branch  which  further  contributes  to  the  freedom  of  the  circulation. 

The  Ophthalmic  Artery  is  a  collateral  branch  of  the  internal  maxil- 
lary. It  enters  the  cranial  cavity  from  the  orbit  by  the  internal  orbital 
foramen,  along  with  the  nasal  branch  of  the  ophthalmic  nerve,  and 
divides  into  meningeal  and  nasal  branches. 

The  Meningeal  Branches  of  opposite  sides  give  off  branches  to  the 
dura  mater,  and  then  unite  to  form  a  single  trunk  which  joins  the 
middle  cerebral  arteries. 

The  Nasal  Branch  passes  through  the  cribriform  plate  to  gain  the 
nasal  chamber. 

The  Sympathetic  Nerve.  Two  branches  from  the  superior  cervical 
ganglion  accompany  the  internal  carotid  artery,  and  anastomose  around 
it  to  form  the  carotid  plexus.  Within  the  cavernous  sinus  they  form 
another  plexus — the  cavernous  plexus.  From  these  plexuses  filaments 
pass  to  join  the  3rd,  4th,  6th,  and  ophthalmic  cranial  nerves.  A 
twig  also  joins  the  large  supei-ficial  petrosal  nerve  from  the  7th,  to 
form  the  vidian  nerve ;  another  passes  to  the  lenticular  ganglion,  either 
separately  or  with  the  ophthalmic  nerve ;  and  some  filaments  pass  to 
the  Gasserian  ganglion. 

The  Brain,  or  Encephalon,  consists  of  four  principal  parts,  viz.,  the 
medulla  oblongata,  the  pons  Varolii,  the  cerebellum,  and  the  cerebmm. 
The  medulla  is  the  division  which  is  in  direct  continuity  behind  with 
the  spinal  cord.  The  pons  projects  as  a  thick  ti'ansvei-se  bar,  or  ridge,  in 
front  of  the  medulla.  The  cerebellum  is  superposed  to  both  medulla 
and  pons.  The  cerebrum  lies  in  front  of  the  other  three  segments,  and 
is  larger  than  these  taken  together.  The  weight  of  the  whole  brain  in 
an  average-sized  horse  is  about  twenty-three  ounces. 

the   medulla   oblongata,  or   bulb   (plates    35   AND   36). 

The  medulla  oblongata  is  continuous  at  the  foramen  magnum  with  the 
spinal  cord,  of  which  it  appears  to  be  the  expanded  anterior  termination. 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  239 

It  rests  by  its  inferior  face  on  the  basilar  process  of  the  occipital  bone ; 
and  its  superior  face,  which  is  concealed  by  the  cerebellum,  is  depressed 
and  forms  the  floor  of  the  4th  ventricle.  Its  anterior  extremity  is  limited 
by  the  pons  Varolii,  and  is  its  widest  part.  The  middle  line  of  the 
medulla  above  and  below  is  traversed  by  lines  which  continue  forwards 
the  superior  and  inferior  median  fissures  of  the  cord. 

The  medulla  is  composed  of  both  white  and  grey  nerve  matter.  The 
former  occurs  at  the  exposed  surface  of  the  medulla,  and  its  nerve 
fibres  are  for  the  most  part  longitudinal  in  direction,  and  are  collected 
into  tracts,  or  bundles.  Thus,  lying  at  each  side  of  the  inferior  median 
fissure  of  the  organ,  there  is  a  tract  termed  the  inferior  pyramid.  To  the 
outer  side  of  this  again,  and  isolated  from  it  by  a  faint  longitudinal 
groove,  is  a  tract  occupying  the  position  of  the  olivary  faxciculus  and 
olivary  body  of  human  anatomy.  More  externally  placed  than  the  last, 
and  forming  a  thick  cord  at  each  side  of  the  medulla,  is  the  restiform 
body ;  while  above  the  restiform  body,  and  nearer  the  superior  median 
fissure,  is  a  more  slender  column  of  fibres  termed  the  miperior  jyyramid. 
The  line  of  separation  between  the  two  last-mentioned  tracts  is  very 
faint,  and  in  the  horse  there  is  seldom  or  never  any  surface  line  of  demar- 
cation between  the  restiform  body  and  the  olivaiy  fasciculus. 

Where  the  medulla  joins  the  cord,  the  inferior  pyramids  become 
narrow,  and  the  inferior  median  fissure  shallow  or  nearly  obliterated  ; 
and  at  that  point  there  is  a  visible  crossing  of  fibres  from  one  side  to 
the  other,  constituting  the  decussation  of  the  j^yramids.  Towards  the 
posterior  part  of  the  medulla  its  lateral  aspect  is  crossed  by  superficial 
curved  fibres — the  arciform  fibres,  and  immediately  behind  the  pons 
Varolii  there  is  a  band  of  transverse  fibres  termed  the  trapezium.  Within 
the  medulla  some  fibres  pass  across  the  median  plane  and  connect  its 
right  and  left  halves. 

The  gi-ey  matter  of  the  medulla  oblongata  occurs  in  considerable 
amount  at  the  floor  of  the  4th  ventricle,  where  it  will  subsequently 
be  exposed. 

Course  of  the  Longitudinal  Fibres  of  the  Medulla  Oblongata. 
The  inferior  pyramid  is  in  part  composed  of  fibres  from  the  inferior 
column  of  the  same  side  of  the  cord,  but  principally  of  fibres  crossing 
from  the  opposite  side  of  the  cord  at  the  decussation.  These  decus- 
sating fibres  are  furnished  mainly  by  the  lateral  column,  but  partly 
also  from  the  superior  column.  The  fibres  of  the  inferior  pyi-amid  are 
continued  through  the  pons  to  the  cerebrum. 

The  olivary  fasciculus  of  fibres  is  derived  from  the  inferior  column 
of  the  cord  on  the  same  side,  and  it  is  continued  through  the  pons  to 
the  cerebrum. 

The  restiform  body  derives  its  fibres  from  all  three  columns  of  the 
cord  on  the  same  side,  but  in  greatest  proportions  from  the  superior 


240  THE  ANATOMY  OF  THE  HORSE. 

column.  It  enters  the  cerebellum,  of  which  it  foiTUS  the  posterior  peduncle. 
The  superior  pyramid  derives  its  fibres  from  the  innermost  part  of  the 
superior  column  of  the  cord  on  the  same  side,  and  its  fibres  are  continued 
through  the  pons  to  the  cerebrum. 

The  medulla  oblongata  shows  the  superficial  origin  of  the  last  seven 
cranial  nerves. 

THE    PONS    VAROLII    (PLATE   33). 

The  pons  Varolii  rests  on  the  basilar  process,  in  front  of  the  medulla 
oblongata.  In  front  of  it  the  crura  cerebri  appear.  Its  inferior 
face  is  convex  in  both  directions,  and  has  a  faint  median  fuiTow. 
The  superior  face  forms  the  anterior  part  of  the  floor  of  the  4th 
ventricle.  Its  extremities  are  curved  upwards  to  enter  the  cere- 
bellum, of  which  they  form  the  middle  peduncles.  The  pons  consists 
of  white  and  grey  nerve  matter.  The  nerve  fibres  of  the  white 
matter  are  airanged  in  two  sets — a  transverse  and  a  longitudinal. 
The  transverse  fibres  consist  of  the  surface  fibres  of  the  pons,  and 
of  deeper  fibres  separated  from  these  by  the  longitudinal  set.  It  is 
these  transverse  fibres  that  curve  upwards  at  either  extremity  of 
the  pons  to  enter  the  cerebellum  as  its  middle  peduncle,  and  they 
accordingly  play  the  part  of  a  commissiire  to  the  right  and  left 
halves  of  the  cerebellum.  The  longitudinal  fibres  are  the  forward 
continuation  of  the  longitudinal  fibres  of  the  medulla  oblongata, 
minus  the  restiform  bodies.  In  front  of  the  pons  these  longitudinal 
fibres  are  continued  as  the  crura  cerebri.  The  grey  matter  of  the 
pons  occurs  Avithiu  its  substance,  and  at  the  floor  of  the  4th  ventricle. 
To  a  group  of  pigmented  nerve  cells  in  the  latter  position,  the  term 
locus  cceruleus  is  applied. 

The  pons  shows  the  superficial  origin  of  the  5th  cranial  nerve,  by  two 
distinct  roots  springing  from  its  lateral  aspect. 

THE  CEREBELLUM  (PLATES  34  AND  35). 

The  cerebellum  is  superposed  to  the  medulla  and  pons,  and 
lies  under  the  supra-occipital  division  of  the  occipital  bone.  The 
tentorium  cerebelli  arches  downwards  in  front  of  it,  and  isolates 
it  fi-om  the  posterior  extremities  of  the  cerebral  hemispheres.  It  is 
traversed  in  the  antero-posterior  direction  by  two  shallow  grooves, 
which  divide  it  into  a  middle  and  two  lateral  lobes.  The  middle 
lobe  is  the  smallest,  and  is  known  as  the  vermiform  lobe.  When 
followed  forwards,  the  vermiform  lobe  is  seen  to  be  reflected  round 
the  anterior  aspect  of  the  cerebellum  to  gain  its  lower  surface  at 
the  roof  of  the  4th  ventricle  ;  and  it  terminates  by  a  blunt  end 
about  the  middle  of  this  surface.  When  followed  posteriorly,  the 
vermiform  lobe  behaves  in  the  same  way,  terminating  at  the  roof  of 
the  4th  ventricle  by  a  blimt  end  opposed  to  the  first.     These  reflected 


DISSECTION    OF    THE    BRAIN,    OR    ENCBPHALON.  241 

portions  have  a  distinct  resemblance  to  two  caterpillars,  and  they  may 
be  distinguished  as  the  anterior  and  jyosterior  vermiform  jyrocesses.  The 
anterior  vei'miform  process  is  adherent  to  the  valve  of  Vieussens.  Each 
lateral  lobe  is  joined  on  its  inferior  aspect  by  three  bundles  of  nerve 
fibres,  which  are  termed  the  ^:)efZimc^es.  The  2)osterior  peduncle  is  the 
termination  of  the  restiform  body,  the  middle  peduncle  is  the  reflected 
extremity  of  the  pons,  and  the  anterior  pednncle  passes  forwards  beneath 
the  corpora  quadrigemina. 

Besides  the  grooves  which  divide  the  cerebellum  into  its  lobes, 
numerous  smaller  fissures  occur  over  its  surface,  and  divide  the  lobes 
into  folia,  or  leaflets.  The  arrangement  of  these  leaflets  will  be  made  much 
more  evident  by  making  an  antero-posterior  vertical  section,  at  or  near 
the  mesial  plane  of  the  organ.  The  peduncles  are  to  be  cut  as  they 
enter  the  lower  face  of  the  lateral  lobe,  and  the  anterior  vermiform 
process  is  to  be  carefully  separated  from  the  valve  of  Vieussens  Avith  the 
scalpel.  This  will  enable  one  half  or  a  little  more  of  the  cerebellum  to 
be  removed  after  the  manner  of  Plate  35. 

The  cerebellum  contains  both  grey  and  white  matter.  The  white 
matter  forms  a  large  mass  in  the  interior,  and  from  this  mass  large  plates 
are  given  off"  towards  the  surface.  From  these  primary  plates  proceed 
more  numerous  smaller  secondary  plates,  and  these  again  detach  small 
terminal  plates  which  end  in  the  surface  folia.  In  consequence  of  this 
disposition  of  the  white  matter,  it  presents  on  vertical  section  a  strikingly 
arborescent  appearance,  to  which  the  term  arbor  vitm  is  applied.  The 
nerve  fibres  of  the  white  matter  are  for  the  most  part  directly  continuous 
with  the  peduncles;  but  some  are  proper  to  the  organ,  and  comiect 
different  points  of  the  grey  matter. 

The  grey  matter  of  the  cerebellum  is  spread  over  its  surface,  and  also 
forms  two  independent  masses  within  the  central  mass  of  white  matter. 
These  latter  have  the  form  of  a  corrugated  capsule,  and  each  is  placed  a 
little  to  one  side  of  the  mesial  plane,  and  is  known  as  the  corptis  dentatum 
of  the  cerebellum.  The  surface  layer  of  gi'ey  matter  invests  the  core 
of  white  matter  within  each  leaflet,  and  also  extends  across  the  bottom 
of  the  fissures  between  adjacent  leaflets.  It  consists  of  two  strata  :  an 
outer  grey  layer,  and  an  inner  rust-coloured  layer. 

The  Fourth  Ventricle  (Plates  35  and  36).  This  is  a  space  between 
the  cerebellum  above,  and  the  medulla  and  pons  below.  Its  boundaries 
are  as  follows : — Its  floor  is  formed  by  the  medulla  and  pons  ;  its  roof 
by  the  valve  of  Vieussens,  the  under  suface  of  the  vermiform  lobe,  and 
the  reflection  of  pia  mater  from  the  medulla  to  the  cerebellum  ;  laterally 
it  is  bounded  in  its  anterior  third  by  the  anterior  peduncle  of  the  cere- 
bellum, and  in  its  posterior  two-thirds  by  the  restiform  body.  The 
widest  part  of  the  space  is  at  the  point  where  the  peduncles  enter  the 
cerebellum,  and  it  contracts  towards  both  extremities.     At  the  posterior 

r 


242  THE    ANATOMY    OF    THE    HORSE. 

extremity  there  is  a  miuute  hole,  which  is  the  entrance  to  the  short  tube 
that  continnes  the  central  canal  of  the  cord  into  the  posterior  end 
of  the  medulla.  The  pointed  posterior  end  of  the  space  is  the 
calamvs  scriptorius  of  human  anatomy,  so  named  from  its  resemblance  to 
a  \\Titing  pen.  The  anterior  end  of  the  space  lies  under  the  valve  of 
Vieussens,  and  leads  into  the  aqtieduct  of  Syhius,  which,  is  acanal  tunnelled 
beneath  the  corpora  quadrigemina,  and  opening  anteriorly  into  the  3rd 
ventricle.  The  floor  of  the  cavity  is  traversed  by  a  longitudinal  mesial 
fuiTOw,  and  it  shows  the  grey  matter  of  the  medulla  and  pons.  The 
cavity  is  lined  by  a  ciliated  epithelium,  and  it  communicates  by  one 
or  more  minute  apertures  in  its  floor  with  the  sub-arachnoid  space.  On 
each  side  of  the  cavity,  between  the  cerebellum  and  the  restiform  body, 
there  is  a  thickened  piece  of  pia  mater — the  choroid  plexus  of  the  Jfth 
ventAcle. 

The  Valve  of  Vieussens  is  a  delicate,  translucent  fold,  placed  at  the 
anterior  part  of  the  roof  of  the  4th  ventricle.  The  lateral  edges  of 
the  valve  are  fixed  to  the  anterior  cerebellar  peduncles,  its  anterior  edge 
is  attached  behind  the  testes,  and  its  posterior  edge  stretches  across 
the  anterior  vermiform  process.  The  upper  face  of  the  valve  is 
adherent  to  the  anterior  veimiform  process,  and  its  lower  face  is  free 
and  foiTos  the  anterior  part  of  the  roof  of  the  4th  ventricle.  The  4th 
nerve  arises  in  the  valve,  close  behind  the  testes,  the  right  and  left 
nerves  appearing  continuous  with  one  another  across  the  middle  line. 

the  cerebrum. 

Under  the  term  cerebnim  are  included  all  the  parts  of  the  encepha- 
lon  except  the  medulla,  pons,  and  cerebellum.  It  forms  a  mass  larger 
than  these  taken  together,  although  the  amount  by  which  it  ex- 
ceeds them  is  much  less  in  the  horse  than  in  man.  The  inferior 
aspect  of  the  cerebral  mass  is  termed  its  base,  and  the  student  should 
begin  by  examining  the  objects  to  be  seen  there  (Plate  33). 

The  Crura  Cerebri  are  two  thick,  round,  white  cords,  which  appear 
in  front  of  the  pons.  At  this  point  they  are  close  together ;  but  as  they 
proceed  forwards,  they  diverge  and  foim  the  posterior  boundaries  of  a 
lozenge-shaped  area — the  interpeduncular  space,  which  is  completed  in 
front  by  the  optic  tracts  and  commissure.  Anteriorly  each  crus 
disappears  into  the  cerebral  hemisphere,  but  its  point  of  termination  is 
concealed  by  the  optic  tract.  The  crus  is  composed  of  a  superficial  and 
a  deep  layer  of  nerve  fibres  with  an  intermediate  thin  stratum  of  grey 
matter.  The  superficial  layer  of  fibres  is  known  as  the  crusta,  and  the 
deep  is  tenxied  the  tegmentum.  The  fibres  of  both  layers  are  continuous 
posteriorly  with  the  longitudinal  fibres  of  the  pons  ;  and  they  are  trans- 
mitted in  front  to  the  optic  thalami,  corpora  striata,  and  grey  matter 
of  the  hemisphere.     The  corpora  quadrigemina,  which  are  superposed 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  243 

to  the  crura,  also  recede  some  fibres.  Tlie  grey  matter  of  the  crus 
contains  nerve  cells  with  dark  pigment,  and  is  therefore  termed  the 
locus  niger.  ^  The  crura  cerebri  show  the  superficial  origin  of  the  3rd 
pair  of  nerves. 

The  Optic  Tracts.  These  are  two  white  cords  of  nerve  fibres  which 
turn  round  the  crura  cerebri,  and  pass'  forwards  and  inwards  to  meet  in 
the  middle  line  and  form  by  their  fusion  the  optic  commissure  or  chiasma. 
This  commissure  rests  on  the  sphenoid  bone,  in  front  of  the  pituitary 
fossa ;  and  in  front  it  gives  off  the  diverging  optic  or  2nd  nerves.  The 
optic  tracts  form  the  anterior  boundary  of  the  interpeduncular  space. 

The  Pons  Tarini  is  the  grey  matter  in  the  posterior  angle  of  the 
interpeduncular  space.  It  is  also  known  as  the  locus  jierf or atus  posticus, 
from  its  being  penetrated  by  numerous  vessels. 

The  Corpus  Albicans  is  a  pea-like,  white  nodule  placed  on  the  middle 
line,  about  the  centre  of  the  interpeduncular  space.  As  will  subse- 
quently be  learned,  the  body  is  formed  by  the  reflection  of  the  anterior 
pillars  of  the  fornix. 

The  Tuber  Cinereum  is  a  layer  of  grey  matter  between  the  corpus 
albicans  and  the  optic  commissure.  It  is  perforated  in  its  centre,  and 
connected  to  the  upper  surface  of  the  pituitary  gland  by  a  hollow  tube 
of  grey  matter — the  infundihuhmi. 

The  Pituitary  Body  is  a  reddish-yellow,  disc-shaped  body,  having  a 
diameter  about  equal  to  that  of  a  sixpence.  It  is  thickest  in  its  centre 
and  thinnest  at  its  rim.  Its  lower  face  rests  on  the  sella  turcica  of  the 
sphenoid  bone ;  and  its  upper  face  receives  the  insertion  of  the  infundi- 
bulum,  and  covers  the  tuber  cinereum,  and,  in  part,  the  corpus  albicans 
and  optic  commissure.  Within  its  striicture  it  comprises  cells  resembling 
thos^  of  the  blood-vascular  or  ductless  glands,  and  others  that  resemble 
nei"vte  cells.  In  the  foetus  it  is  proportionally  larger,  and  contains  a 
cavity  which  communicates  with  the  3rd  ventricle  through  the  infundi- 
bulum. 

The  pons  Tarini,  corpus  albicans,  and  tuber  cinereum  form  the  floor 
of  the  3rd  ventricle,  a  cavity  Avhich  the  dissector  will  hereafter  expose 
by  working  from  the  upper  aspect  of  the  cerebrum. 

The  Lamina  Cinerea,  or  Lamina  Terminalis,  is  a  thin,  delicate  layer 
of  grey  matter  which  is  placed  above  and  in  front  of  the  optic  commis- 
sure.    It  is  the  anterior  boundary  of  the  3rd  ventricle. 

The  Locus  Perforatus  Anticus  is  a  spot  of  grey  matter  at  each  side 
of  the  optic  commissure,  penetrated  by  numerous  vessels  for  the  corpus 
striatum,  which  lies  above  the  spot. 

The  Fissure  of  Sylvius,  is  a  faint  and  ill-defined  groove  Avhich  begins 
at  the  locus  perforatus  anticus,  and  extends  outwards  across  the  hemi- 
sphere. 

The  Great  Longitudinal  Fissure.     In  front  of  the  optic  chiasma  the 


24-1-  THE  ANATOMY  OF  THE  HORSE. 

cerebral  mass  is  scon  to  be  mesially  divided  by  the  gi-eat  longitudinal 
fissure.  This  fissure,  as  will  be  better  seen  when  the  brain  is  viewed 
from  above,  is  a  great  vertical  mesial  cleft  extending  the  whole  length  of 
the  cerebrum,  which  it  partially  divides  into  right  and  left  halves,  or 
hemispheres. 

The  Olfactory  Bulbs.  The  olfactory  bulb  is  the  white  body  situated 
at  the  anterior  end  of  the  hemisphere.  It  occupies  the  olfactory  fossa 
at  the  forepart  of  the  cranial  cavity;  and  unless  special  care  is  taken  in 
the  removal  of  the  brain,  the  bulb  is  apt  to  be  separated  from  the  hemi- 
sphere and  left  in  that  fossa.  From  the  free  surface  of  the  bulb  the 
delicate  filaments  of  the  olfactory  (1st  cranial)  nen^e  pass  through  the 
criliriform  plate  of  the  ethmoid  bone,  and  enter  the  nasal  chamber.  The 
bulb  is  hollow,  having  a  centi'al  cavity  that  is  in  communication  with 
the  anterior  cornu  of  the  lateral  ventricle. 

The  Olfactory  Peduncle  is  a  short,  thick,  white  cord  immediately 
behind  the  bulb,  and  in  direct  continuity  with  it.  The  hemisphere  is 
slightly  depressed  over  the  peduncle,  the  depression  being  termed  the 
olfactory  fissure.  The  peduncle  divides  posteriorly  into  the  olfiictory 
tracts. 

The  Olfactory  Tracts.  These  are  two  white  diverging  bands — an 
inner  and  an  outer — that  continue  the  olfactory  peduncle  backwards. 
The  intenml  tract  (inner  olfactory  root)  is  short,  and  passes  backwards 
and  inwards  to  the  edge  of  the  great  longitudinal  fissure.  The  external 
tract  (outer  olfactory  root)  is  a  much  longer  band  which  curves  out- 
wards and  backwards  across  the  fissure  of  Sylvius,  and  then  encircles 
outwardly  the  uncinate  and  hippocampal  convolutions,  to  reach  the 
tentorial  aspect  of  the  hemisphere,  on  which  it  is  lost.  At  the  fissure 
of  Sylvius  the  tract  seems  to  lose  some  of  its  fibres  in  front  of  the  unci- 
nate convolution,  and  behind  that  point  it  becomes  grey  on  its  surface. 

Behind  the  angle  of  divergence  of  the  olfactory  tracts  is  a  smooth  and 
slightly  convex  area — the  quadrilateral  space  of  Paul  Broca.  The  surface 
layer  of  this  space  consists  of  grey  matter  constituting  the  middle  or 
greT/  olfactory  root;  and,  according  to  Broca,  it  covers  white  fibres  that 
connect  the  olfactory  bulb  to  the  crus  cerebri  and  to  the  anterior  cere- 
bral commissure  (anterior  white  commissure  of  3rd  ventricle). 

If  now  the  olfactory  peduncle  be  raised  from  the  olfactory  fissure,  it 
will  be  seen  to  be  connected  to  the  frontal  lobe  of  the  hemisphere  by 
a  lamina  termed  by  Broca  the  sujyei^ior  olfactory  root.  This  lamina  is 
grey  on  its  surface  and  white  beneath,  and  if  it  be  ruptured  the  cavity 
of  the  bulb  and  the  communication  between  that  cavity  and  the  anterior 
cornu  of  the  lateral  ventricle  will  be  brought  into  view. 

External  to  the  outer  olfactory  tract,  each  hemisphere  shows  numerous 
winding  worm-like  ridges,  termed  convolutions,  and  internal  to  the  pos- 
terior half  of  the  same  tract  there  is  seen  a  thick  ridge — the  hippo- 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  245 

campal  convolution — which  terminates  behind  the  fissure  of  Sylvius 
in  a  nipple-like  eminence — the  unci7iat€  convohdion  (mastoid  lobule,  or 
mammillary  eminence).  These  will  presently  be  more  particularly 
described. 

Directions. — The  student  nuist  now  reverse  the  position  of  the  brain, 
laying  it  with  its  base  downwards,  while  he  proceeds  to  examine  its 
upper  aspect. 

The  Great  Longitudinal  Fissure  is  now  seen  in  its  entirety.  It 
extends  from  the  anterior  to  the  posterior  end  of  the  cerebrum,  and 
appears  to  completely  separate  the  right  and  left  hemispheres.  In  the 
natural  state  the  fissure  is  occupied  by  the  falx  cerebri.  Gently 
separate  the  contiguous  margins  of  the  hemispheres,  so  as  to  widen  out 
the  fissure.  Except  towards  the  hinder  end  of  the  fissure,  this  proceed- 
ing requires  no  dissection,  but  at  that  point  the  hemispheres  are  united 
on  the  middle.  It  is,  however,  a  mere  adhesion  through  the  medium  of 
pia  mater.  Separate  the  hemispheres  here  by  traction,  or  by  cutting 
carefully  in  the  mesial  plane.  There  will  now  be  exposed  (Fig.  31)  a 
white  body — the  corpus  callostim — which  connects  the  hemispheres  at 
the  bottom  of  the  great  longitudinal  fissure.  At  the  same  time  there 
will  be  brought  into  view  the  opposed  inner  s\irfaces  of  the  liemispheres. 
The  Cerebral  Convolutions.  In  his  examination  of  the  base  of  the 
brain,  and  more  clearly  now,  the  student  will  have  observed  that  the 
siu'face  of  the  hemisphere  is  not  smooth,  but  traversed  by  numeroiis 
winding  worm-like  elevations.  These  are  termed  the  cerebral  convohi- 
tions  or  gyri;  and  the  intermediate  grooves  or  fissures  are  technically 
termed  sulci. 

At  first  sight  it  might  be  supposed,  as  indeed  was  believed  until  a 
comparatively  recent  date,  that  the  disposition  of  these  convolutions  is 
quite  iiTCgular  and  hap-hazard.  Observation  has  shown,  however,  that 
such  is  far  from  being  the  case,  and  that  the  convolutions  have  a  nearly, 
if  not  altogether  constant,  arrangement.  In  the  human  subject,  indeed, 
the  surface  of  the  hemisphere  has  been  accurately  mapped,  and  each 
convolution  named.  In  the  brain  of  the  horse  the  plan  of  these  convo- 
lutions appears  to  be  as  uniform  as  in  man ;  and  althoiigh,  perhaps,  the 
convolutions  are  not  absolutely  identical  in  any  two  brains,  or  even  in 
the  two  hemispheres  of  the  same  brain,  still  the  irregularities  are  so 
slight  as  to  permit  one  to  describe  with  considerable  minuteness  what 
might  be  termed  a  common  plan.  The  mapping  of  the  surface  of  the 
hemispheres  derives  its  chief  interest  and  vitility  from  the  discovery  that 
definite  areas  are  associated  with  particular  functions,  in  such  a  way  that 
when  these  areas  are  destroyed  or  injm-ed  there  follows  total  loss  or  dis- 
turbance of  these  functions,  and  that  in  some  cases  the  exercise  of  par- 
ticular functions  can  be  brought  about  by  applying  stimuli  to  particular 
spots  of  the  cerebral  cortex. 


246  THE  ANATOMY  OF  THE  HORSE. 

In  tlie  brain  of  man  the  hemisi)here  is  primarily  subdivided  into  five  lobes,  viz. ,  frontal, 
parietal,  occipital,  temporo-sphenoidal,  and  central,  the  last  being  also  known  as  the 
Island  of  Eeil,  or  the  Insula.  The  lines  of  separation  between  these  lobes  are  certain 
well-marked  fissures,  distinguished  from  the  sulci  in  general  by  their  greater  depth  and 
constancy.  In  each  lobe,  again,  the  secondary  sulci  form  the  lines  of  separation  between 
a  definite  number  of  convolutions. 

In  tlie  third  edition  of  Professor  Chauveau's  admirable  work  {Trait,-  d'Anatomie  com- 
2mri'c  clrs  Animaux  domestiqiies)  an  attempt  is  made  to  describe  the  cerebral  convolu- 
tions of  the  horse  after  the  plan  followed  in  human  anatomy,  and  to  establish  an  almost 
comi>lete  correspondence  of  these  parts  in  the  two  brains.  It  appears  to  me,  after  very 
careful  consideration,  that  except  in  a  few  points,  an  identity  between  convolutions 
in  tlie  two  brains  is  not  clearly  indicated  on  anatomical  grounds  alone.  That  most  of 
the  convolutions  of  the  human  brain  have  cori-esponding  convolutions  in  the  brain  of  the 
horse  is  more  than  probable  ;  and  experimental,  patliological,  or  developmental  evidence 
may  yet  place  this  correspondence  beyond  doubt.  In  the  meantime,  however,  and  pro- 
visionally, I  think  it  preferable  to  describe  the  cerebral  convolutions  of  the  horse  accord- 
ing to  what  appears  the  most  natural  plan. 

The  surface  of  each  hemisphere  (excluding  from  present  consideration  its  inner  aspect) 
is  divided  into  three  lobes  or  areas,  viz.,  an  anterior  lobe,  a  postero-superior  lobe,  and  a 
postero-inferior  lobe.  This  subdivision  is  effected  by  certain  fissures  (Plate  34),  as 
follows : — 

1.  The  Crucial  Fissure.  This  is  a  short  fissure  which  begins  near  the  midille  of  each 
hemisphere  where  it  margins  the  great  longitudinal  fissure.  Passing  outwards,  it  joins 
the  great  oblique  fissure.  The  crucial  fissure  separates  the  anterior  from  the  postero- 
superior  lobe.  In  the  right  hemisphere  of  Plate  34  these  lobes  are  connected  across  the 
fissure  by  a  small  annectcnt  or  bridging  convolution. 

2.  The  Gre.vt  Oblique  Fissure.  This  is  the  most  pronounced  fissure  of  the  hemi- 
sphere. Beginning  near  the  middle  of  the  upper  surface  of  the  hemisphere,  where  it  is 
continuous  wtli  the  crucial  fissure,  it  is  directed  obliquely  outwards,  downwards,  and 
backwards,  to  reach  the  tentorial  aspect  of  the  hemisphere  (Plate  33).  It  separates  the 
postero-superior  from  the  postero-inferior  lobe. 

3.  The  Lateral  Fissure.  This  fissure  begins  on  the  upper  surface  of  the  hemisphere, 
at  the  jioint  of  junction  of  the  crucial  and  great  oblique  fissures.  It  curves  round  the 
side  of  the  hemispliere,  with  a  slightly  forward  inclination ;  and  it  separates  the  anterior 
from  the  postero-inferior  lobe. 

4.  The  Fissure  of  Sylvius.*  This  begins  at  the  base  of  the  brain  (Plate  33),  at  the 
side  of  the  optic  commissure.  It  passes  outwards  as  a  faint  and  ill-defined  depression  in 
front  of  the  uncinate  convolution  ;  and  crossing  the  outer  olfactory  tract,  it  divides  into 
four  branches,  which,  however,  are  mere  sulci.  One  of  these  is  directed  backwards  between 
the  outer  olfactory  tract  and  the  postero-inferior  lobe  ;  another  passes  forwards  between 
the  inner  olfactory  tract  and  the  anterior  lobe  ;  a  third  ascends  into  the  postero-inferior 
lobe  ;  and  the  fourth  is  directed  forwards  into  tlie  anterior  lobe.  In  the  first  part  of  its 
course  the  fourth  branch  separates  the  adjacent  convolutions  of  the  anterior  and  postero- 
inferior  lobes,  being  itself  separated  by  a  bridging  convolution  between  these  lobes  from 
the  lower  extremity  of  the  lateral  fissure. 

To  the  outer  side  of  the  outer  olfactory  tract,  at  the  point  from  which  these  branches 
of  the  Sylvian  fissure  radiate,  there  is  a  minute  nodular  convolution  that  is  partially  or 
entirely  concealed  from  view  until  the  adjacent  convolutions  are  slightly  separated. 
This  seems  to  foreshadow  the  convolutions  of  the  insula  of  man. 

The  Anterior  Lobe  presents  four  convolutions  :— 

1.  The  First  Anteriw  Convolution  (Plate  33,  1.  A)  is  seen  on  the  under  surface  of  the 
lobe.  It  lodges  the  olfactory  peduncle  in  the  olfactory  fissure  ;  and  when  the  peduncle 
is  in  position  it  shows  an  inner  and  an  outer  part,  the  former  occupying  the  position  of 
the  (jiirus  rectus  of  human  anatomy. 

2.  The  Second  Anterim-  Convolution  begins  on  the  under  surface  of  the  lobe  (Plate  33, 
*  The  first  part  of  the  fissure  of  Sylvius,  as  far  as  the  outer  olfactory  tract,  is  soiuetimes  anil  more 

<:orrectly  called  the  vvllaj  of  SylvhiK,  ami  Uroca  restricts  the  term  Jissvre  of  Si/lvii'n  to  the  third  of 
the  above-described  branches. 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  247 

2.  A),  external  to  the  preceding.  It  runs  forwards  and  upwards  round  the  extremity  of 
the  hemisphere,  and  abuts  on  tlie  antero-marginal  convolution  (Plate  34). 

3.  The  Third  Anterior  Convolution  begins  at  the  under  surface  of  the  lobe  (Plate  33, 

3.  A)  to  the  outer  side  of  and  behind  the  preceding.  It  curves  upwards  and  forwards 
across  the  hemisiihere,  and  reaches  its  upper  aspect  (Plate  34).  Here  it  is  reflected  back- 
wards and  inwards  ;  and  turning  upon  itself,  it  descends  to  near  the  point  from  which  it 
started,  being  connected  at  its  termination  by  a  bridging  convolution  to  the  first  convolu- 
tion of  the  postero-inferior  lobe. 

4.  The  Antero-marginal  Convolution  (Plate  34,  A.  M.)  lies  at  the  forepart  of  the  great 
longitudinal  fissure,  and  is  visible  on  both  the  upper  and  inner  aspects  of  the  lobe.  Be- 
ginning at  the  crucial  fissure  (being  sometimes  connected  to  the  postero-marginal  convolu- 
tion of  the  postero-superior  lobe),  it  passes  forwards  at  the  edge  of  the  hemisphere,  at  the 
anterior  end  of  which  the  first  and  second  anterior  convolutions  abut  upon  it. 

The  Postero-superior  Lobe  comprises  three  convolutions  : — 

1.  The  Postero-marginal  Convolution  (Plate  34,  T.  M.)  extends  along  the  margin  of  the 
lobe,  appearing  on  both  its  upper  and  inner  surfaces,  and  lying  in  series  with  the  antero- 
marginal  convolution  of  the  anterior  lobe.  Beginning  at  the  crucial  fissure,  it  passes 
backwards  at  the  edge  of  the  hemisphere,  and  reaches  its  tentorial  aspect. 

2.  The  First  Oblique  Convolution  (Plate  34,  \.  O)  lies  external  to  the  preceding.  Be- 
ginning in  front,  near  the  margin  of  the  hemisphere,  it  passes  obliquely  backwards  and 
outwards,  and  curves  round  the  extremity  of  the  lobe  to  reach  its  tentorial  surface. 

3.  The  Second  Oblique  Convolution  (Plate  34,  2.  O)  passes  with  an  oblique  direction 
between  the  preceding  convolution  and  the  great  oblique  fissiure,  and  reaches  the  tentorial 
surface  of  the  lobe  (Plate  33),  from  which  it  seems  to  be  in  part  continued  by  the  outer 
olfactory  tract. 

The  PosTERO-iNPEEiOR  LoBE.  The  sulci  of  this  lobe  are  numerous  and  small,  and  it 
is  difficult  to  divide  it  naturally  into  convolutions.  For  convenience  of  description,  how- 
ever, two  convolutions  may  be  recognised  in  it : — 

1.  The  First  Postero-inferior  Convolution  (Plates  33  and  34,  1.  P.  I.)  is  four-sided,  and 
contains  within  itself  several  short  sulci.  It  lies  behind  the  third  convolution  of  the 
anterior  lobe,  to  which  it  is  connected  by  a  bridging  convolution. 

2.  The  Second  Postero-inferior  Convolution  (Plates  33  and  34,  2.  P.  I.)  lies  at  the 
posterior  part  of  the  lobe,  above  the  outer  olfactory  tract ;  and  its  posterior  extremity 
appears  on  the  tentorial  surface  of  the  hemisphere.  Like  the  preceding,  it  possesses 
numerous  minor  sulci  within  itself. 

Directions. — There  still  remains  for  examination  the  inner  surface  of  each  hemisphere. 
Sejiarate  the  hemispheres  as  widely  as  possible  along  the  great  longitudinal  fissure.  At 
the  upper  edge  of  this  fissure  there  will  now  be  seen  the  inner  aspect  of  the  antero-marginal 
and  postero-marginal  convolutions  already  described,  and  between  the  lower  edge  of  these 
and  the  corpus  callosum  there  lies  a  thick  convolution — the  gyrus  fornicatus. 

The  Gyrus  Fornicatus  (Fig.  31). — This  is  comi>arable  to  a  lobe,*  rather  than  to  a 
convolution.  It  is  disposed  in  a  great  curve,  or  arch,  from  which  it  is  named.  It  begins 
at  the  forepart  of  the  under  siu'face  of  the  hemisphere,  in  front  of  the  lamina  cinerea, 
and  here  it  is  narrow  and  pointed.  It  bends  round  the  anterior  extremity  (genu)  of  the 
corpus  callosum,  acquiring  at  its  point  of  reflection  a  great  increase  in  thickness.  It 
passes  backwards  above  the  corpus  callosum,  and  below  the  antero-marginal  and  postero- 
marginal  convolutions.  From  the  former  body  it  is  separated  by  the  fissure  of  the  corpus 
callosum,  while  the  calloso-marginal  fissure  (great  limbic  fissure  of  Broca)  separates  it  from 
the  marginal  convolutions  above.  In  this  part  of  its  coiirse  the  gyrus  is  distinctly  divided 
into  two  tiers  by  a  fissure  that  traverses  it  in  its  length.  Posteriorly  this  fissure  becomes 
very  shallow,  and  the  gyrus,  losing  its  double  character,  turns  roimd  the  posterior  end 
(splenimn)  of  the  corpus  callosum  and  reaches  the  tentorial  surface  of  the  hemisphere. 
At  this  point  it  becomes  slightly  constricted  ;  and  after  being  connected  with  the  convolu- 

*  Paul  Broca  (Anatomic  comparce  ihs  circonrohUions  cerebndes)  considers  that  this  part  of  the 
hemisphere  represents  not  merely  a  lobe,  but  several  lobes — that  it  is,  in  fact,  the  eoLuivalent  of  all 
the  rest  of  the  cerebral  cortex.  He  accordingly  divides  the  surface  of  the  hemisphere  primarily  into 
two  great  divisions — the  great  liinbic  lobe  (gyrus  fornicatus)  and  the  conrolutionary  mass. 


248 


THE  ANATOMY  OF  THE  HORSE. 


tions  of  the  postero-siiperior  and  postero-inferior  lobes,  it  is  directed  forwards  at  the  base 
of  the  brain  (Plate  33,  Hipp,  con.),  between  the  cms  cerebri  and  the  outer  olfactory 
tract.     Finally,  it  terminates  in  the  nipple-like  eminence  already  noticed  (Unc.  con.). 

The  whole  of  tliis  great  convolution  corresponds  very  closely  to  the  gyrus  fornicatus  of 
human  anatomy.  Tlius,  the  part  which  turns  round  the  genu  and  rests  on  the  upper 
surface  of  the  corpus  collosum  is  tlie  caUosal  convolution ;  the  part  from  the  splenium 
to  the  side  of  the  cerebral  crus  is  the  hippocampal  convolution;  and  the  nipple-like 
eminence  is  the  uncinate  convolution. 

The  hippocami)al  part  of  the  gyrus  fornicatus  lias  a  small  process  which  projects 
forwards  under  the  splenium  ;  and  as  the  convolution  curves  forwards  to  emerge  at  the 
side  of  tlie  crus,  it  rests  on  the  optic  thalamus.  By  carefully  raising  the  convolution  from 
the  thalamus,  there  will  be  brouglit  into  view  a  fissure  on  the  under  asi)ect  of  the  former. 
This  is  the  hippocampal  Jigs  are,  and  it  projects  tlie  convolution  into  the  lateral  ventricle 
as  the  hippocampus.  Beyond  this  fissure  tlie  edge  of  the  liippocamjius  is  seen,  margined 
by  a  thin-edged  white  band — tlie  taenia  hippocampi.  The  hippocampus  and  its  taenia 
here  form  the  upper  boundary  of  the  r/rcat  transverse  fissure  of  the  brain,  by  wliich  the 
pia  mater  of  the  hemisphere  projects  towards  the  interior  of  the  cerebrum  as  the  velum 
intcrpositum. 


Fig.  31. 
Corpus  Callosum  and  Inner  Face  of  the  Cerebral  Hemisphere. 


1,  2.  Right  and  left  cerebral  hemispheres ;  3.  Cerebellum  ;  C.  Corpus  callosum  ;  G.  F.  Gyrus 
fornicatus  (its  callosal  portion) ;  C.  M.  Calloso-marginai  fissure ;  A.  M.  Antero-marginal  convolu- 
tion ;  P.  M.  Posteromarginal  convolution. 

Directions. — With  a  large,  thin-bladed,  sharp  knife  a  horizontal  slice 
shonld  be  removed  from  the  top  of  one  or  both  cerebral  hemispheres, 
down  to  the  level  of  the  corpus  callosiim. 

The  hemisphere  will  now  be  seen  to  contain  both  grey  and  white 
matter.  In  the  centre  of  the  hemisphere  the  white  matter  forms  a  large 
mass  connected  with  that  of  the  opposite  side  by  the  corpus  callosum. 
At  the  surface  the  mass  sends  a  white  core  into  each  convolution.     The 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  249 

great  sheet  of  grey  raatter  on  the  surface  of  the  hemisphere  invests 
the  white  core  in  each  conA^olution,  and  also  extends  across  the  bottom 
of  each  fissure. 

The  Corpus  Callosum  (Plate  35  and  Fig.  31)  is  a  great  commissure  of 
nerve  fibres  connecting  the  right  and  left  hemispheres.  It  termin- 
ates behind  in  a  thickened  max-gin — the  sjylenium;  and  in  front  it  is 
abruptly  bent  downwards  and  backwards,  the  bend  being  named  the 
genu,  and  the  reflected  portion  the  rostrum.  The  rostrum  becomes 
narrower  as  it  descends,  and  is  connected  to  the  optic  commissure  by  the 
lamina  cinerea.  Along  the  middle  line  of  its  lower  face  the  corpus 
callosum  is  connected  posteriorly  with  the  fornix,  and  anteriorly  with 
the  septum  lucidum ;  and  on  each  side  it  forms  the  roof  of  a  cavity  in 
the  hemisphere — the  lateral  ventricle.  Nearly  all  the  fibres  of  the 
corpus  callosum  have  a  transverse  direction ;  but  on  each  side  of  the 
longitudinal  middle  line  of  its  vipper  face  there  are  a  few  longitudinal 
fibres  termed  the  striae  longitudinales,  or  nerves  of  Lancisi. 

Directions. — If  the  corpus  callosum  be  now  cut  through  in  the  longi- 
tudinal direction,  a  little  to  one  side  of  the  middle  line,  and  dissected 
outwards,  the  lateral  ventricle  will  be  exposed.  The  corpus  callosum, 
it  will  now  be  seen,  is  thickest  at  its  posterior  extremity,  and  thinnest  at 
its  middle. 

The  Lateral  Ventricles  (Plate  35)  are  two  in  number,  one  in  each 
hemisphere.  They  are  separated  from  one  another  along  the  middle 
line  by  the  fornix  and  septum  lucidum,  but  beneath  the  former  body  they 
communicate  through  the  foramen  of  Monro.  The  central  portion  of 
each  cavity  is  termed  the  body,  and  its  prolongations  before  and  behind 
are  termed  respectively  the  anterior  and  the  descending  cornu. 

On  the  floor  of  the  body  of  the  cavity  the  following  objects  will 
be  noticed : — In  front,  a  large  pear-shaped  grey  eminence — the  corpus 
striatum ;  behind,  another  body  of  about  the  same  size  but  white  on  its 
surface — the  hippocampus;  between  the  corpus  striatum  and  the 
hippocampus,  a  groove,  in  which  there  lies  a  red  granular  cord — the 
choroid  plexus.  Where  the  hippocampus  bounds  this  groove,  it  is 
margined  by  a  white  band — the  tamia  hippocampi ;  and  if  the  choroid 
plexus  be  pulled  gently  backwards,  another  white  band  will  be  seen  to 
margin  the  corpus  striatum  where  it  bounds  the  groove— this  is  the 
tcenia  semicircularis.  * 

The  anterior  cornu  is  occupied  by  the  base  of  the  corpus  striatum. 
It  curves  downwards  and  forwards  into  the  anterior  part  of  the 
hemisphere,  where  it  communicates  with  the  cavity  of  the  olfactory  bulb. 

The  descending  cornu  contains  the  prolongations  of  the  hippocampus 
and  its  taenia.      It  passes  at  first  backwards  and  outwards,  and  then 

*  The  optic  thalamus  and  tsenia  semicircularis  are  generally  enumerated  among  the  objects 
visible  in  the  body  of  the  lateral  ventricle.  In  the  brain  of  the  horse,  however,  the  choroid  plexus 
completely  conceals  from  view  the  optic  thalamus,  and  in  most  cases  also  the  taenia  semicircularis. 


250  THE  ANATOMY  OF  THE  HORSE. 

ciu-A-es  downwards,  forwards,  and  inwards,  terminating  at  the  base  of  the 
hemisphere  in  the  uncinate  convolution.  The  ventricles  are  lined  by  a 
ciliated  epithelium,  whicli  is  continuous  through  the  foramen  of  Monro 
with  the  lining  of  the  3rd  ventricle. 

The  Septum  Lucidum  is  a  thin,  translucent  partition  between  the  two 
lateral  ventricles.  It  is  broadest  in  front,  where  it  is  attached  to  the 
rostiTim,  or  reflected  part  of  the  corpus  callosum.  Its  upper  edge  is 
attached  to  the  coi-pus  callosum,  and  its  lower  edge  to  the  fornix  ;  and 
posteriori}-  these  edges  meet  at  an  acute  angle.  The  septum  consists 
of  white  matter  in  its  centre,  with  a  layer  of  grey  matter  on  each  side. 
In  man  it  contains  a  small  isolated  cavity — the  5th  ventricle. 

The  Fornix,  or  arch,  is  a  mesially  placed  white  band,  consisting  of  a 
central  part,  or  body,  and  two  paii-s  of  processes,  or  pillars.  The  body 
is  flattened  above  and  below,  and  broadest  behind.  Its  upper  face  is 
adherent  posteriorly  to  the  corpus  callosum,  but  in  front  it  dips  down 
and  leaves  beneath  the  forepart  of  the  corpus  callosum  a  space  occupied 
by  the  septiim  lucidum.  The  under  surface  of  the  body  rests  on  the 
velum  interpositum,  and  at  its  anterior  extremity  arches  over  the 
foramen  of  Monro.  The  anterior  ]nnars  of  the  fornix  are  two  white 
cords  which  descend  in  front  of  the  foramen  of  Monro,  being  separated 
by  a  slight  interval.  Reaching  the  base  of  the  brain,  they  turn  on 
themselves,  forming  thus  the  corpus  albicans,  and  they  then  enter  the 
optic  tlialamus.  The  posterior  pillars  are  broader  and  flatter,  and  not 
so  well  defined.  Each  in  part  bestows  its  substance  on  the  surface  of  the 
hippocampus,  and  in  part  it  descends  along  the  anterior  edge  of  that 
body  as  the  taenia  hipjiocampi,  or  corpus  fimhriatum. 

The  Corpus  Striatum  (Plates  35  and  36).  This  is  the  large  grey 
body  already  noticed  in  the  body  and  anterior  comu  of  the  lateral 
ventricle.  In  shape  it  is  pyriform,  having  its  broad  end  directed 
forwards  and  inwards,  and  its  tapering  end  backwards  and  outwards  to 
the  roof  of  the  descending  comu.  The  body  comprises  two  masses  of 
grey  matter,  separated  from  each  other  by  intermediate  white  fibres 
which  curve  upwards  and  outwards  from  the  cerebral  crus.  The  upper 
mass  of  grey  matter — termed  the  nncleus  caudatus — is  that  which  pro- 
jects into  the  lateral  ventricle.  The  lower  mass — the  nucleus  lenticu- 
laris — lies  above  the  quaclrilatercd  space  already  seen  at  the  base  of  the 
hemisphere  between  the  diverging  olfactory  tracts. 

The  T.ENLv  Sbmicircularis  (Plate  36)  is  a  narrow  white  band  that 
extends  between  the  corpus  striatum  and  the  optic  thalamus.  (Its 
relation  to  the  thalamus  will  be  better  seen  in  the  next  stage  of  the  dis- 
section.) 

The  Hippocampus  is  the  curved  eminence  already  noticed  in  the  body 
and  descending  horn  of  the  lateral  venti'icle.  It  rests  on  the  optic 
thalamus,  from  which  it  is  separated  by  the  velum  interpositum.      The 


DISSECTION    OF    THE    BRAIN,    OR    BNCEPHALON.  251 

ventricular  aspect  of  the  body  is  white,  but  the  surface  that  rests  on  the 
optic  thalamus  is  grey.  The  hippocampus  is  to  be  viewed  as  a  convolu- 
tion of  the  cerebnun,  being,  in  fact,  an  inward  projection  of  the  hippo- 
campal  convolution  already  noticed  (page  248). 

Directions. — The  corpus  callosum  and  septum  lucidum  should  be  cut 
away  in  order  to  see  the  upper  surface  of  the  fornix,  which  should  then 
be  divided  transversely  in  its  middle.  The  anterior  part  should  be 
raised  forwards  and  upwards  to  expose  its  anterior  pillars  and  the 
foramen  of  Monro.  The  posterior  part  should  be  removed  along  with 
the  hippocampus  in  order  to  bring  into  view  the  velum  interpositum. 
The  dissection  will  then  assume  the  form  of  Plate  36  ;  but  the  optic 
thalami  and  the  pineal  gland,  there  exposed,  will  be  covered  by  the 
velum. 

The  Velum  Interpositum  is  a  triangular  fold  of  pia  mater,  continuous 
by  its  base  with  the  pia  mater  on  the  hinder  end  of  the  cerebrum.  Its  apex 
lies  at  the  foramen  of  Monro,  and  its  lateral  edges,  fringed  by  the  chor- 
oid plexuses,  project  towards  the  lateral  ventricle  through  what  is  termed 
the  great  transverse  fissure  of  the  cerebrum.  This  is  aia  arched  cleft  extend- 
ing over  the  optic  thalami,  from  the  extremity  of  the  descending  horn  on 
one  side  to  the  same  point  on  the  other.  Above  it  is  bounded  centrally 
by  the  fornix,  and  on  each  side  by  the  hippocampus  and  its  taenia. 
The  velum  interpositum  is,  like  the  pia  mater  in  general,  a  vascular 
membrane ;  and  the  choroid  plexus  of  each  side  is  a  thickened 
and  highly  vascular  portion  of  it.  Along  its  centi-e  the  veins  of  Galen 
extend  backwards,  and  unite  to  turn  round  the  posterior  extremity  of 
the  corpus  callosum,  and  enter  the  inferior  longitudinal  sinus.  The 
velum  should  now  be  raised  from  its  apex  backwards,  when  it  will  be 
seen  to  cover  the  optic  thalami  and  the  pineal  gland,  and  care  must  be 
taken  lest  the  latter  be  removed  with  it. 

The  Optic  Thalami.  Each  of  these  is  a  large  grey-coloured  body, 
superposed  to  the  crus  cerebri  behind  the  corpus  striatum,  and  in  front 
of  the  corpora  quadrigemina.  Its  upper  surface  is  convex  and  covered 
by  the  velum  interpositum.  When  followed  outwards  this  surftice 
changes  its  direction,  looking  backwards  and  downwards ;  and  it  there 
forms  part  of  the  boundary  of  the  descending  horn  of  the  lateral 
ventricle.  Inwardly  the  right  and  left  thalami  are  opposed  to 
one  another  along  the  middle  line,  and  they  include  between  them 
the  3rd  ventricle.  In  front  each  thalamus  is  separated  from  the 
corpus  striatum  by  a  groove,  in  which  will  now  be  seen  more  distinctly 
the  taenia  semicircularis.  Behind,  another  groove  isolates  the  thalamus 
from  the  nates. 

The  Pineal  Gland  is  a  small,  reddish,  conical  body,  named  from  its 
resemblance  to  a  pine  cone.  It  stands  by  its  base  on  the  middle  line 
between  the  optic  thalami  and  the  nates.     From  its  base  two  white 


252  THE  ANATOMY  OF  THE  HORSE. 

bands — the  peduncles  of  the  pineal  gland — extend  forwards  along  tlie 
gi'oove  between  the  two  thalami ;  and  at  the  foramen  of  Monro  each 
pednncle  luiites  with  the  anterior  pillar  of  the  fornix  to  descend  to  the  base 
of  the  brain,  and  concnr  in  forming  the  corpus  albicans.  In  structure 
the  body  presents  some  resemblance  to  lymphoid  tissue,  but  it  also 
contains  some  branched  corpuscles  which  are  possibly  nerve  cells. 
Imbedded  in  it  is  a  quantity  of  gritty  calcareous  matter  termed  the 
acet'vulus  cerebri,  or  brain-sand. 

The  Third  Ventricle  is  a  narrow  space  whose  sides  are  formed  by 
the  optic  thalami.  Its  floor  corresponds  to  the  parts  already  examined 
in  the  interpeduncular  space,  viz. ,  the  pons  Tarini,  corpus  albicans,  and 
tuber  cinereum.  Its  roof  is  formed  by  the  velum  interpositum  covered 
by  the  fornix.  In  front  it  is  bounded  by  the  lamina  cinerea,  and  it  here 
communicates  with  the  lateral  ventricles  by  the  foramen  of  Monro. 
Posteriorly  the  aqueduct  of  Sylvius  enters  it  from  the  4th  ventricle. 
The  cavity  is  crossed  by  three  commissures  :  1.  The  Anterior  Commis- 
sure is  a  small  white  cord  of  nerve  fibres  stretching  transversely  between 
the  corpora  striata  at  the  anterior  end  of  the  cavity,  and  immediately  in 
front  of  the  descending  anterior  pillars  of  the  fornix.  The  fibres  of  the 
commissure  are  traceable  through  the  corpora  striata  into  the  white 
matter  of  the  hemispheres.  2.  The  Middle  (soft)  Cominissure  is  com- 
posed of  delicate  grey  matter  cementing  the  inner  surfaces  of  the  thalami, 
and  apt  to  be  more  or  less  niptured  in  handling  the  brain.  3.  The 
Posterior  Commissure  is  white,  like  the  anterior ;  and  its  fibres  connect 
the  two  thalami  at  the  base  of  the  pineal  body,  and  immediately  in  front 
of  the  nates. 

The  3rd  ventricle  has  a  ciliated  lining  continuous  with  that  of  the 
4th  throiigh  the  aqueduct  of  Sylvius,  and  with  that  of  the  lateral 
ventricles  through  the  foramen  of  Monro.  In  the  foetiis  the  cavity  com- 
municates through  the  tuber  cinereum  and  infundibuhim  with  tlic 
pituitary  body. 

The  Foramen  of  Monro,  or  Foramen  Commune  Anterius,  is  the  common 
point  of  communication  between  the  3rd  and  lateral  ventricles.  It 
might  be  desci-ibed  as  a  short  vertical  shaft  ascending  from  the  fore- 
part of  the  3rd  ventricle,  and  opening  under  the  fornix,  which  is  thrown 
over  it  like  an  arch.  Beneath  this  arch  the  lateral  ventricles  communi- 
cate with  one  another  and  with  the  3rd  ventricle. 

The  Corpora  Quadrigemina  are  two  pairs  of  bodies  superposed  to  the 
crura  cerebri  behind  the  optic  thalami.  The  anterior  pair  of  bodies,  or 
Tuites,  are  larger  than  the  posterior  pair,  or  testes,  from  which  they  are 
separated  by  a  groove.  Between  the  right  and  left  nates  there  is  a  well- 
defined  groove,  but  the  groove  between  the  testes  is  faint  or  not  observ- 
able. The  nates  are  grey  on  their  surface,  but  the  testes  are  white. 
The  bodies  were  named  nates  and  testes  from  a  fancied  resemblance 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  253 

to  the  hips  and  testicles  of  a  man,  bnt  these  terms  are  far  from  express- 
ing the  rehxtive  size  of  the  two  bodies. 

The  Aqueduct  of  Sylvius,  or  iter,  is  a  tunnel  which,  commencing 
posteriorly  in  the  4th  ventricle,  beneath  the  valve  of  Vieusseus, 
extends  forwards  beneath  the  corpora  quadrigemina,  and  opens  into  tlie 
hinder  part  of  the  3rd  ventricle.  It  possesses  a  ciliated  lining  con- 
tinuous with  that  of  the  ventricles  which  it  connects. 

Optic  Tracts  and  Corpora  Geniculata.— The  optic  tracts  have  already 
been  seen  at  the  base  of  the  brain,  where  they  form  the  anterior 
boundaries  of  the  interpeduncular  space.  When  followed  backwards, 
each  tract  will  be  found  to  turn  round  the  cms  cerebri,  and  join  the 
optic  thalamus.  At  the  point  of  junction  two  eminences  are  placed,  an 
outer,  or  anterior,  and  an  inner,  or  posterior.  These  are  named  respectively 
the  corpus  geniculatum  externwn  and  internum.  They  are  composed  of 
grey  matter  from  which  some  fibres  of  the  optic  ti'act  pass.  Other 
fibres  of  the  tract  come  directly  from  the  optic  thalamus,  and  others 
from  the  corpora  quadrigemina. 

the  cranial  or  encephalic  nerves  (plate  33). 
In  the  examination  of  the  base  of  the  brain,  the  roots  of  the  cranial 
nerves  have  already  been  noticed,  but  it  will  be  advantageous  to  describe 
them  here  as  a  series.  The  cranial  nerves  are  distinguished  by  special 
names,  and  also  by  numerical  designations.  It  must  be  observed,  how- 
ever, at  the  outset  that  there  are  two  difierent  systems  of  enumeration 
in  use  among  anatomists,  the  first  of  which  recognises  twelve,  and  the 
other  nine,  pairs  of  nerves.  This  diversity  of  nomenclature  is  apt  to  lead 
to  confusion,  but  fortunately  this  confusion  does  not  extend  to  veterinary 
anatomy,  in  which,  both  at  home  and  abroad,  the  first  and  more  natural 
of  these  methods  is  exclusively  employed.  This  system  is  also  that 
employed  by  human  anatomists  on  the  continent,  but  by  British  human 
anatomists  the  number  of  cranial  nerves  is  stated  as  nine  pairs.  The 
following  table  exhibits  in  the  central  column  the  special  names  of 
the  nerves,  and  in  the  side  columns  their  numerical  designations  under 
the  two  systems  : — 

1st  pail-       .         .     Olfactory  nerves 1st  pair. 

2iid   ,,         .         .     Optic  nerves  ......     2nd  ,, 

3rd    ,,         .         .     Oculo-motor  nerves        ....     3rd   ,, 

4tli    ,,         .         .     Pathetic  or  Trochlear  nerves  .         .     4th   „ 

5th    ,,         .         .     Trifacial  or  Trigeminal  nerves       .         .     5th   ,, 

6th    ,,         .         .     Abducent  nerves 6th   ,, 

7th    , ,         .         .     Facial  nerves  (Portio  dura)    .         .         .  | 
8th    „         .         .     Auditory  nerves  (Portio  mollis)    .         .  )  " 

9th    ,,         .         .     Glosso-pharyngeal  nerves      .         .         .  -, 
10th    ,,         .         .     Pneumogastric  or  Vagus  nerves    .         .  K%i\x 
11th    ,,         .         .     Spinal  Accessory  nerves        .         .         .  j 
12th    ,,         .         .     Hypoglossal  nerves        ....     9th   ,, 

The  Olfactory  or  1st  nerve.    The  fibres  of  this  nerve  leave  the  surface 


254  THE  ANATOMY  OF  THE  HORSE. 

of  the  olfactory  bulb,  and  pass  through  the  foramina  of  the  cribriform 
plate  to  reach  the  summit  of  the  nasal  chamber.  They  are  there  distri- 
buted in  the  olfactory  division  of  the  lining  membrane  of  that  chamber. 
The  Optic  or  2nd  nerve  arises  from  the  optic  chiasma  or  commisHure, 
and  reaches  the  back  of  the  orbit  by  passing  through  the  optic  foramen. 
Piercing  the  sclerotic  and  choroid  tunics  of  the  eyeball,  its  fibres  radiate 
outwards  and  form  one  of  the  layers  of  the  retina.  As  already  seen,  the 
optic  chiasma  is  formed  by  the  fusion  of  the  optic  tracts,  each  of  which 
derives  its  fibres  from  the  optic  thalamus,  corpora  geniculata,  corpora 
quadrigemina,  and  decussation  of  the  pyramids.  In  the  optic  chiasma 
some  of  the  fibres  of  each  tract  cross  and  are  continued  in  the  optic 
nerve  of  the  opposite  side.  Some  of  the  fibres  of  each  optic  tract,  it  is 
stated,  cross  in  the  chiasma  and  return  to  the  brain  by  the  opposite 
tract,  while  in  the  same  way  fibres  pass  from  the  one  optic  nerve  to  the 
other  optic  nerve. 

The  OcuLO-AiOTOR  or  3rd  nerve  arises  from  the  inner  side  of  the  crus 
cerebri  by  a  number  of  bundles,  the  fibres  of  which  are  traceable  to 
nerve  cells  in  the  corpora  quadrigemina.  The  nerve  leaves  the  cranium 
by  the  foramen  lacerum  orbitale,  and  reaches  the  orbit. 

The  Pathetic,  Trochlear,  or  4th  nerve  appears  to  arise  in  the  valve 
of  Vieussens  (Plate  35).  Some  of  its  fibres  are  decussate  with  those  of 
the  opposite  nerve,  and  the  others  are  traceable  to  nerve  cells  of  the 
locus  cseruleus,  or  of  the  corpora  quadrigemina.  Emerging  from  the 
valve,  the  nerve  winds  round  the  crus  cerebri,  and  appears  in  front  of 
the  pons.  It  leaves  the  cranium  by  the  minute  pathetic  foramen,  and 
reaches  the  back  of  the  orbit.     It  is  the  smallest  of  the  cranial  nei'ves. 

The  Trifacial,  Trigeminal,  or  5th  nerve  springs  out  of  the  side  of 
the  pons  by  two  roots.  The  outer  and  larger  of  these  is  termed  the 
sensory  root;  and  its  fibres  are  traceable  to  cells  of  the  grey  matter  of 
the  medula,  pons,  and  locus  cseruleus,  and  possibly  also  to  the  cerebellum. 
This  root  near  its  origin  expands  into  a  large  ganglion — the  Gasserian 
ganglion,  beyond  which  it  divides  into  three  branches,  viz.,  the  ophthal- 
mic, superior  maxillary,  and  inferior  maxillary  divisions.  The  inner  or 
motor  root  of  the  5th  nerve  is  traceable  to  grey  matter  of  the  pons.  It 
joins  the  inferior  maxillary  division  of  the  sensory  root.  The  superior 
maxillary  division  leaves  the  cranium  by  the  foramen  rotundum,  the 
ophthalmic  division  by  the  foramen  lacerum  orbitale,  and  the  inferior 
maxillary  division  by  the  forepart  of  the  foramen  lacerum  basis  cranii. 
The  trifacial  is  the  largest  of  the  cranial  nerves. 

The  Abducent  or  6th  nerve.  This  nerve  springs  from  the  anterior 
part  of  the  medulla,  in  line  with  the  faint  groove  that  limits  outwardly 
the  inferior  pyramid.  Some  of  its  fibres  issue  from  the  groove  between 
the  pons  and  the  medulla,  while  others  penetrate  the  trapezium.  The 
fibres  of  the  nerve  are  traceable  to  a  group  of  nerve  cells  in  the  medulla. 


DISSECTION    OF    THE    BRAIN,    OR    ENCEPHALON.  255 

The  nerve  reaches  the  orbit  by  passing  through  the  foramen  lacerum 
orbitale. 

The  Facial  or  7th  nerve  springs  out  of  the  medulla,  close  behind 
the  pons,  its  fibres  seeming  to  continue  outwards  the  trapezium.  Its 
rootlets  are  traceable  to  nuclei  of  grey  matter  in  the  medulla.  The 
nerve  is  joined  by  a  delicate  filament — the  portio  intermedia — which 
appears  between  the  roots  of  this  and  the  next  nerve.  The  7th  nerve 
enters  the  internal  auditory  meatus  in  company  with  the  8th  nerve. 
Separating  from  that  nerve,  it  passes  along  a  canal  in  the  petrous 
temporal  bone — the  aqueduct  of  Fallopius — from  which  it  emerges  by 
the  stylo-mastoid  foramen,  under  the  parotid  gland.  Within  the  aque- 
duct of  Fallopius  the  nerve  forms  a  knee-shaped  bend,  and  at  that  point 
it  shows  a  minute  ganglion — the  geniculate  ganglion — from  which  pro- 
ceed the  great  and  small  superficial  petrosal  nerves  (pages  189  and  214). 

The  Auditory  or  8th  nerve  springs  from  the  medulla,  close  behind 
the  pons,  and  immediately  external  to  the  root  of  the  7th.  It  is  here 
compounded  of  two  roots — a  superior  and  an  inferior.  The  siqyerior  root 
(Plate  35)  passes  over  the  restiform  body  to  the  grey  matter  at  the  floor 
of  the  4th  ventricle.  The  inferior  root  springs  out  of  the  side  of  the 
restiform  body,  its  fibres  arising  from  nerve  cells  of  that  body  or  of  the 
grey  matter  at  the  floor  of  the  4th  ventricle,  and  possibly  also  from  the 
cerebellum.  The  8th  nerve  enters  the  intenial  auditory  meatus,  and 
penetrates  to  the  internal  ear. 

The  Glosso-pharyngeal  or  9th  nerve  springs  out  of  the  side  of  the 
medulla,  a  little  behind  the  outer  extremity  of  the  trapezium.  It  is 
here  compounded  of  two  or  three  bundles,  the  outermost  being  in  line 
with  the  roots  of  the  next  two  nerves.  The  fibres  emanate  from  nerve 
cells  of  the  grey  matter  at  the  floor  of  the  4th  ventricle.  The  nerve 
leaves  the  cranium  by  the  posterior  part  of  the  foramen  lacerum  basis 
cranii,  and  at  that  point  it  shows  a  minute  ganglion — the  petrous 
ganglion,  or  the  ganglion  of  Andersch,  from  which  the  nerve  of  Jacobson 
arises  (page  269). 

The  Pneumogastric,  Vagus,  or  10th  nerve  is  formed  by  a  number  of 
rootlets  which  spring  from  the  side  of  the  medulla,  behind  and  in  line 
Avith  the  outermost  fibres  of  the  9th  nerve.  Its  fibres  arise  from  nerve 
cells  of  the  medulla.  The  nerve  passes  out  of  the  cranium  by  the 
posterior  part  of  the  foramen  lacerum  basis  cranii,  and  is  joined  by  the 
inner  division  of  the  11th  nerve.  As  the  nerve  passes  through  the 
foramen  it  presents  an  enlargement — the  upper  ganglion,  or  ganglion  of 
the  root.  From  this  ganglion  arises  the  auricular  branch  of  the  vagus, 
which  penetrates  to  the  aqueduct  of  Fallopius,  Avhere  it  anastomoses 
Avith  the  7th  nerve ;  afterwards  emerging  from  the  bone  in  company 
with  that  nerve,  to  be  distributed  to  the  mucous  membrane  of  the 
external  auditory  process. 


256  THE    ANATOMY    OF    THE    HOR«E. 

The  Spinal  Accessory  or  11th  nerve  comprises  two  sets  of  roots — a 
spinal  and  a  medullary.  The  spinal  roots  appear  along  the  lateral 
column  of  the  cervical  part  of  the  spinal  cord,  in  which  they  arise  from 
a  group  of  nerve  cells  towards  the  middle  of  the  grey  ci-escent.  By  the 
union  of  these  roots  there  is  formed  a  coi'd  which  travels  upwards 
between  the  superior  and  the  inferior  roots  of  the  cersdcal  spinal  nerves, 
becoming  thicker  as  it  ascends.  This  cord  enters  the  cranial  cavity  by 
the  foramen  magnum,  and  is  then  joined  by  the  medullary  roots.  The 
medullary  roots  spring  out  of  the  side  of  the  medulla  oblongata,  behind 
and  in  line  with  the  roots  of  the  10th  nerve,  the  fibres  arising  from 
nerve  cells  at  the  floor  of  the  4th  ventricle.  These  roots  join  the  spinal 
part  of  the  nerve,  which  then  leaves  the  cranium  by  the  foramen  lacemm 
basis  cranii,  along  with  the  10th  nerve.  In  the  foramen  of  exit  the 
trunk  of  the  nerve  resolves  itself  into  two  portions — an  internal  and  an 
external.  The  i^iternal  portion  joins  the  10th  nerve  ;  the  external  piortion 
is  that  which  has  already  been  seen  in  the  dissection  of  the  neck  (page 
151). 

The  Hypoglossal  or  12th  nei-ve  is  formed  by  the  fusion  of  rootlets 
that  spring  from  the  lower  face  of  the  medulla,  along  the  line  that  indi- 
cates the  outer  limit  of  the  inferior  pyramid.  These  roots  are  in  series 
with  the  inferior  roots  of  the  spinal  nerves ;  and  sometimes  there  is  also 
present  a  superior  root,  in  series  with  the  superior  roots  of  the  same 
nerves,  and  provided  with  a  minute  ganglion.  These  roots  arise  from 
nerve  cells  of  the  medulla.  The  roots  of  the  nerve  perforate  the  dura 
mater,  and  unite  in  emerging  from  the  cranium  by  the  condyloid 
foramen. 


CHAPTER    YIL 

DISSECTION   OF   THE   EYEBALL. 

Directions. — Let  the  student  procure  three  or  four  eyes  of  the  horse, 
or,  failing  these,  of  the  ox.  They  should  be  excised  from  the  orbit 
immediately  after  death,  and  as  much  as  possible  of  the  optic  nerve  should 
be  preserved  in  connection  with  the  eye.  "While  an  assistant  holds 
the  eye  without  squeezing  it,  the  dissector  should  clean  the  optic  nerve 
and  the  o\iter  surface  of  the  sclerotic  with  forceps  and  a  sharp  scalpel. 
One  of  the  eyes  so  prepared  should  be  completely  frozen  in  a  mixture  of 
ice  and  salt,  and  it  should  then  be  bisected  vertically  with  a  large  knife 
or  fine  saw.  While  still  frozen,  the  section  to  which  the  optic  nerve  is 
attached  should  be  fastened  by  a  strong  pin  to  a  layer  of  solid  paraffin 
at  the  bottom  of  a  wide  and  shallow  basin.  It  should  be  fastened  with 
the  cut  surface  upwards,  the  pin  being  passed  vertically  from  the  centre 
of  that  surface ;  and  the  vessel  should  then  be  filled  with  water.  The 
remaining  segment  should  be  laid  on  the  freezing  mixture,  with  its  cut 
sm-face  upwards.  By  an  examination  of  both  segments,  the  student 
should  make  out  the  following  points  : — 

The  Globe  or  Ball  of  the  eye  approaches  the  spherical  in  form,  as  is 
expressed  by  these  designations.  On  closer  inspection,  however,  it  Avill 
appear  to  be  made  iip  of  two  combined  portions  fi-om  spheres  of  different 
sizes.  The  posterior  portion,  forming  about  five-sixths  of  the  ball,  is  a 
sphere  of  comparatively  large  size  with  a  small  segment  cut  off  it  in 
front ;  and  at  this  point  there  is  applied  to  it  the  anterior  portion,  which, 
being  a  segment  of  a  smaller  sphere,  projects  at  the  front  of  the  ball 
with  a  greater  convexity  than  the  posterior  portion. 

The  eyeball  consists  of  concentrically  arranged  coats,  and  of  refracting 
media  enclosed  within  these  coats.     The  coats  are  thi-ee  in  number,  viz., 

(1)  an  external  protective  tunic  made  up  of  the  sclerotic  and  cornea, 

(2)  a  middle  vascular  and  pigmentary  tunic — the  choroid,  (3)  an  internal 
nervous  layer — the  retina.  The  sclerotic  is  the  white  opaque  part  of 
the  outer  tunic,  of  which  it  forms  about  the  posterior  five-sixths,  being 
co-extensive  with  the  larger  sphere  already  mentioned.  The  cornea 
forms  the  remaining  one-sixth  of  the  outer  tunic,  being  co-extensive  with 
the  segment  of  the  smaller  sphere.  It  is  distinguished  from  the 
sclerotic  by  being  colourless  and  transparent.      The  choroid  coat  will  be 


258 


THE  ANATOMY  OF  THE  HORSB. 


recognised  as  the  black  layer  lying  subjacent  to  the  sclerotic.  It  does 
not  line  the  cornea,  but  terminates  behind  the  line  of  junction  of  that 
coat  with  the  sclerotic,  by  a  thickened  edge — the  ciliary  processes.     At 


Fig.  32. 
View  of  the  Lower  Half  of  the  Rioht  Adult  Human  Eve,  divided  horizontally 

THROUGH   THE   MIDDLE.       MAGNIFIED   FOUR   TIMES  (A.   ThomSOil). 

1.  The  cornea  ;  1'.  Its  conjunctival  layer  ;  2.  The  sclerotic ;  2'.  Sheath  of  the  optic  nerve  i)ass)ng 
into  the  sclerotic ;  3.  3'.  The  choroid  ;  4.  Ciliary  muscle,  its  radiating  portion  ;  4'.  Cut  fibres  of  the 
circular  portion ;  5.  Ciliary  fold  or  process ;  0.  Placed  in  the  posterior  division  of  the  aqueous 
chamber,  in  front  of  the  suspensory  ligament  of  the  lens  ;  7.  The  iris  (outer  or  temporal  side) ;  "'.  The 
smaller,  inner,  or  nasal  side  ;  8.  Placed  on  the  divided  optic  nerve,  points  to  the  arteria  centralis 
retinae;  8'.  Papilla  optica  at  the  passage  of  the  optic  nerve  into  the  retina;  8".  Fovea  centralis 
retinae ;  r.  The  nervous  Liyer  of  the  retina  ;  /.  The  bacillary  layer  ;  9.  Ora  serrata,  at  the  com- 
mencement of  the  ciliary  part  of  the  retina  ;  10.  Canal  of  Petit  ;  11.  Anterior  division  of  the 
aqueous  chamber,  in  front  of  the  pupil  ;  12.  The  crystalline  lens,  within  its  capsule  ;  13.  The  vitreous 
humour  ;  a.  a.  i>.  Parts  of  a  line  in  the  axis  of  the  eye  ;  0.  b.  b.  b.  A  line  in  the  transverse  diameter. 


DISSECTION   OF   THE   EYEBALL.  259 

the  line  of  junction  of  the  sclerotic  and  cornea,  the  iris  passes  across  the 
interior  of  the  eye.  This,  which  may  be  viewed  as  a  dependency  of  the 
choroid,  is  a  muscular  curtain  perforated  by  an  aperture  termed  the 
pupil.  The  retina  will  be  recognised  as  a  delicate  glassy  layer,  lining 
the  greater  part  of  the  choroid. 

The  refracting  media  of  the  eye  are  three  in  number,  viz.,  (1)  the 
aqueous  humour — a  watery  fluid  enclosed  in  a  chamber  behind  the  cornea  ; 
(2)  the  crystalline  lens  (and  its  capsule) — a  transparent  soft  solid  of  a 
biconvex  form,  and  placed  behind  the  iris ;  (3)  the  vitreous  humour — a 
transparent  material  with  a  consistence  like  thin  jelly,  and  occupying  as 
much  of  the  interior  of  the  eye  as  is  subjacent  to  the  choroid. 

Directions. — Another  eye  should  be  cleaned  like  the  first,  and 
used  for  the  more  particular  examination  of  the  sclerotic  and  cornea. 

The  Sclerotic  is  a  strong,  opaque  fibi'ous  membrane  which  in  great 
measure  maintains  the  form  of  the  eyeball,  and  protects  the  more 
delicate  structures  within  it.  Its  anterior  portion,  which  is  covered  by 
the  ocular  conjunctiva,  is  visible  in  the  undissected  eye,  and  is  commonly 
known  as  the  "white  of  the  eye."  In  form  it  is  bell-shaped,  and  the 
optic  nerve  pierces  it  behind  like  a  handle.  The  point  of  perforation, 
however,  is  not  exactly  at  the  centre  of  the  summit  of  the  bell,  but  a 
little  to  its  inner  side.  When  the  nerve  is  cvit  oft'  close  to  the  sclerotic, 
the  nerve-bundles  appear  as  if  passing  through  the  apertures  of  a  sieve, 
and  to  this  appearance  the  term  lamina  cribrosa  is  applied.  The  sheath 
of  the  nerve  passes  on  to  the  sclerotic  around  the  point  of  perforation. 
In  front  the  rim  of  the  bell  becomes  continuous  with  the  coniea.  The 
outer  surface  of  the  membrane  receives  the  insertion  of  the  muscles  of 
the  eyeball.  The  inner  surface  (which  will  afterwards  be  exposed)  is  of 
a  light  brown  colour,  and  is  connected  to  the  choroid  by  fine  processes 
of  connective-tissue — the  lamina  fusca.  The  coat  is  thickest  over  the 
posterior  part  of  the  eyeball,  and  is  thinnest  a  little  behind  its  junction 
with  the  cornea. 

Structure. — The  sclerotic  is  composed  of  connective-tissue,  there  being 
a  great  preponderance  of  white  fibres,  but  intermixed  with  these  are 
some  fine  elastic  fibres.  The  bundles  of  fibres,  which  are  disposed  both 
meridionally  and  equatorially,  have  a  felted  an-angement,  but  the 
surface  fibres  are  mostly  longitudinal.  The  texture  of  the  sclerotic  is 
only  slightly  vascular,  the  capillaries  forming  a  wide-meshed  network. 
It  is  most  vascular  just  behind  the  cornea. 

The  Cornea  is  the  anterior  transparent  portion  of  the  outer  coat  of 
the  eyeball.  It  may  be  viewed  as  a  part  of  the  sclerotic  specially 
modified  to  permit  the  passage  of  light  into  the  interior  of  the  eye.  Its 
outline  is  elliptical  approaching  the  circular,  and  its  greatest  diameter 
is  transverse.  At  its  periphery  it  joins  the  sclerotic  by  continuity  of 
tissue ;  and  as  the  edge  of  the  cornea  is  slightly  bevelled,  and  has  the 


260  THE  ANATOMY  OF  THE  HORSE. 

fibrous  sclerotic  carried  for  a  little  distance  forward  on  its  outer  surface, 
the  cornea  is  generally  said  to  be  fitted  into  the  sclerotic  like  a  watch- 
glass  into  its  rim.  The  venous  canal  of  Schlemm  runs  circularly  around 
the  eyeball  at  the  line  of  junction  of  the  sclerotic  and  cornea.  The 
anterior  surface  of  the  coniea  is  exquisitely  smooth,  and  is  kept  moist 
by  the  lachrymal  secretion.  Its  posterior  surface  forms  the  anterior 
boundary  of  the  chamber  in  which  the  aqueous  humour  is  contained. 
The  cornea  is  of  unifoi-m  thickness ;  and,  as  will  afterwards  be  proved 
in  removing  it,  it  is  very  difficult  to  cut,  being  of  a  dense,  almost 
homy  consistence.  When  its  normal  convexity  is  disturbed,  the  cornea 
becomes  opaque. 

Strtidvre. — Save  a  few  capillary  loops  at  its  margin,  the  cornea  is 
without  vessels.  Its  structure  comprises  the  following  layers,  which  are 
enumerated  in  order  from  the  anterior  to  the  posterior  surface  : — 

1.  The  Anterior  Epithelmm  is  a  stratified,  pavement  epithelium, 
continuous  at  the  margin  of  the  cornea  with  the  conjunctival  epithelium. 

2.  The  Anterior  Elastic  Lamina  (Bowman's  membrane).  This  is  a 
structureless,  elastic  layer.  It  is  extremely  thin  in  the  eye  of  the 
lower  animals,  but  is  better  developed  in  the  human  eye. 

3.  The  Substantia  Propria.  This,  which  forms  the  main  thickness  of 
the  cornea,  is  composed  of  fibrous  connective-tissue  arranged  in  lamellse 
parallel  to  the  surfaces  of  the  cornea.  Between  adjacent  lamelhe  there 
is  left  a  network  of  spaces  and  branching  canals,  in  which  are  found  the 
branched  corneal  corptiscles. 

4.  The  Posterior  Elastic  Lamina  (Descemet's  membrane)  is  a  thick, 
structureless,  elastic  layer. 

5.  The  Posterior  Epithelium  is  a  single  layer  of  polygonal  cells. 
Directions. — A  sti'ong  pin  should  now  be  passed  through  the   optic 

nerve,  and  used  to  fasten  the  eye  beneath  the  surface  of  water  in  a 
wide  and  shallow  vessel,  as  already  directed  in  the  case  of  the  frozen 
section.  While  one  hand  steadies  the  eye  beneath  the  -svatei*,  an  incision 
is  to  be  made  with  the  other  through  the  cornea,  using  for  the  purpose 
a  very  sharp  scalpel.  As  soon  as  the  incision  is  made,  some  of  the 
aqueous  humour  Avill  escape  into  the  Avater,  and  may  possibly  be  recog- 
nised by  a  slight  inky  discoloration,  which  is  due  to  a  2^ost-mortem 
disintegration  of  the  pigmented  epithelium  lining  the  cavity  in  which 
the  humour  is  contained.  Still  keeping  the  eye  under  water,  one  blade 
of  a  pair  of  small  scissors  should  be  introduced  Avithin  the  incision,  and 
the  coniea  should  be  excised  immediately  in  front  of  its  junction  Avith 
the  sclerotic.  The  iris  will  by  this  means  be  exposed,  and  the  next  step 
must  be  to  remoA-e  a  portion  of  the  sclerotic  so  as  to  expose  the  sub- 
jacent choroid.  Beginning  at  its  anterior  edge,  it  may  be  incised  back- 
Avards  tOAvards  the  optic  nerve,  snipping  it  bit  by  bit  Avith  the  point  of 
the  scissors.     Another  incision  may  then  be  made  parallel  to  the  first. 


DISSECTION    OF    THE    EYEBALL. 


261 


and  about  half  an  inch  from  it.  The  piece  of  sclerotic  between  the 
incisions  may  then  be  raised  and  turned  backwards  by  destroying  the 
slender  processes,  nerves,  and  vessels  that  connect  it  to  the  choroid.  At 
the  anterior  edge  of  the  piece  of  choroid  thus  exposed,  and  immediately 
behind  the  rim  of  the  iris,  there  will  be  seen  a  whitish  zone — the  ciliary 
body,  or  anmdus  alhidus. 

The  Aqueous  Humour  occupies  a  chamber  which  is  bounded  in  front 
by  the  posterior  surface  of  the  cornea ;  and  behind  by  the  capsule  and 
suspensory  ligament  of  the  lens,  and  by  the  ends  of  the  ciliary  processes. 
It  is  across  this  chamber  that  the  iris  extends,  and  the  chamber  is  some- 
times described  as  being  divided  by  the  iris  into  two  compartments,  viz., 
an  anterior,  in  front  of  the  iris  j  and  a  posterior,  behind  it.  In  the 
living  eye,  however,  the  posterior  surface  of  the  iris  contacts  with  the 
lens-capsvile,  so  as  to  leave  only  a  narrow  chink  behind  the  attachment 
of  the  curtain  to  which  the  term  posterior  chamber  may  be  applied. 
The  aqueous  humour  is  composed  of  water  with  a  small  proportion  of 
common  salt  in  solution. 

The  Iris  is  a  muscular  pigmented  cui*tain  extended  across  the  interior 
of  the  eye,  and  having  about  its  centre  an  aperture  termed  the  jmjnl. 
By  variations  in  the  size 
of  this  aperture,  the 
amount  of  light  trans- 
mitted to  the  retina  is 
regulated.  It  varies 
somewhat  in  colour,  but 
is  most  frequently  of  a 
yellowish-brown  tint.  Its 
anterior  surface,  which 
shows  some  lines  con- 
verging to  the  pupil,  in 
bathed  by  the  aqueous 
humour,  as  is  also  its 
posterior  surface  immedi- 
ately internal  to  its 
attachment.  The  gi'eater 
part  of  the  posterior  sur- 
face, however,  is  in  con- 
tact with  the  capsule  of 
the  lens,  und  glides  on  it 
during  the  movements  of  the  curtain.  The  circumferential  border  is 
attached  within  the  junction  of  the  sclerotic  and  cornea.  The  inner 
border  circumscribes  the  pupil,  which  varies  in  outline  according  to  its 
size.  When  much  contracted,  the  pupil  is  a  very  elongated  ellipse,  the 
long  axis  of  which  is  in  the  line  joining  the  nasal  and  temporal  canthi  of 


Choroid  membrane  and  Iris  exposed  by  the  removal 
OF  THE  Sclerotic  and  Cornea  (Quain  after  Zinn). 

R.  One  of  the  segments  of  the  sclerotic  thrown  back  ;  h.  Ciliary 
muscle  ;  c.  Iris  ;  e.  One  of  the  ciliary  nerves  ;  /.  One  of  the  vasa 
vorticosa  or  choroidal  veins. 


262  THE  ANATOMY  OF  THE  HORSE. 

the  eyelids;  but  when  it  is  extremely  dilated,  the  ellipse  approaches  the 
circular  in  form.  Appearing  at  the  upper  margin  of  the  pupil,  there  are 
generally  two  or  three  little  sooty  masses  termed  the  corpora  nigra. 
These  are  little  dependent  balls  of  the  iivea,  or  pigmentary  layer  covering 
the  back  of  the  iris. 

Structure. — This  comprises  a  connective-tissue  stroma,  muscular  tissue, 
and  an  anterior  and  a  posterior  epithelium. 

The  Stroma  is  a  framework  of  connective-tissue,  the  fibres  having  a 
radial  arrangement,  and  the  corpuscles  being  branched  and  pigmented. 
The  pigment  varies  in  shade  from  yellow  to  dark  brown  or  almost  black. 
The  Muscular  Tissue  is  of  the  non-striated  variety,  and  its  fibres  are 
arranged  in  two  sets,  viz.,  (1)  the  sphincter  of  the  pupil,  a  narrow  band 
around  the  pupil,  and  close  to  the  posterior  surface  of  the  curtain ;  (2) 
the  dilator  of  the  2iupil,  whose  fibres  begin  at  the  attached  edge  of  the 
curtain,  and  extend  radially  inwards  to  end  in  the  sphincter.  The  size 
of  the  pupil  is  regulated  by  the  state  of  contraction  of  these  two  muscles. 
When  the  action  of  the  sphincter  preponderates,  the  aperture  is  con- 
tracted ;  when  that  of  the  dilator  preponderates,  the  pupil  is  dilated. 

The  Anterior  Epithelium  is  continuous  at  the  attached  edge  of  the 
iris  with  the  posterior  epithelium  of  the  cornea.  It  is  a  single  layer  of 
pigmented  cells. 

The  Posterior  Ejnthelium,  or  Uvea,  comprises  several  layers  of  cells 
similarly  pigmented ;  and,  as  before  stated,  the  corpora  nigra  are  small 
dependent  portions  of  it.  In  the  eyes  of  albinos  the  iris  is  devoid  of 
pigment ;  and  occasionally  in  the  horse  and  dog  the  pigment  is  only 
present  in  spots,  and  the  animal  is  then  said  to  be  "  wall-eyed." 

Vessels. — The  arteries  of  the  iris  are  derived  from  the  ciliary  branches 
of  the  ophthalmic.  They  form  at  the  circumference  of  the  iris  a  larger 
circle,  from  which  radial  vessels  pass  inwards  and  form  around  the 
pupil  a  smaller  circle.  The  veins  have  a  similar  disposition,  and  termin- 
ate in  those  of  the  choroid. 

In  the  foetus  the  pupil  is  closed  by  a  vascular  transparent  membrane 
— the  memlnrina  2iupillaris,  which  disappears  before  birth. 

The  Ciliary  Muscle.  This  is  a  zone  of  non-striated  muscular  tissue 
which  forms  the  outer  layer  of  the  ciliary  body,  and  lies  behind  the 
circumferent  edge  of  the  iris.  It  consists  (1)  of  an  outer  radiating 
set  of  fibres,  which  arise  from  the  inner  surface  of  the  sclerotic  close 
behind  its  line  of  junction  with  the  cornea,  and  pass  backwards  to 
be  inserted  into  the  choroid  and  ciliary  processes  ;  and  (2)  of  an  inner 
circular  set,  which  surround  the  rim  of  the  iris.  When  the  radiating 
fibres  contract,  they  pull  forward  the  choroid  coat  and  ciliary  pro- 
cesses, and  allow  the  lens  to  bulge  forwards  by  slackening  its  tense 
suspensory  ligament.  This  is  the  mechanism  by  which  the  eye  is 
accommodated  for  near  objects. 


DISSECTION    OF   THE   EYEBALL.  263 

The  Choroid  Coat.  This  is  a  bell-shaped,  dark  membrane  which 
lines  the  sclerotic.  Its  outer  surface,  when  exposed  by  the  removal  of 
the  sclerotic,  has  a  shaggy  appearance  due  to  the  tunica  fusca  which 
unites  the  two  coats.  Between  the  two  the  ciliaiy  vessels  and  nerves 
pass  forwards.  The  inner  surface  of  the  choroid  is  lined  by  the  layer 
of  pigmented  hexagonal  cells  belonging  to  the  retina.  Behind  it  is 
pierced  by  the  optic  nerve ;  and  in  front  it  is  contimied  as  the  ciliary 
processes,  which  form,  as  it  were,  the  rim  of  the  bell. 

Directions. — In  the  eye  prepared  to  expose  the  iris  and  choroid,  a 
segment  of  the  former  and  of  the  ciliary  muscle  should  be  carefully  and 
delicately  removed  with  scissors,  so  as  to  lay  bare  a  number  of  the 
ciliary  processes.  This  is  to  be  done  while  the  eye  remains  immersed 
in  water. 

The  Ciliary  Processes.  These  form  a  fringe  around  the  slightly 
inverted  rim  of  the  choroid.  They  number  upwards  of  a  hundred,  and 
each  projects  on  the  inner  side  of  the  rim,  as  a  small  swelling  separated 
by  depressions  from  the  adjacent  processes.  The  outer  surface  of  each 
is  covered  by  the  ciliary  muscle  ;  the  inner  surface  rests  in  a  depression 
on  the  suspensory  ligament  of  the  lens ;  behind  each  is  continuous  with 
the  texture  of  the  choroid ;  and  in  front  it  terminates  in  a  rounded  end 
which  bounds  in  part  the  so-called  posterior  chamber  of  the  aqueous 
humour,  behind  the  peripheral  part  of  the  iris. 

Structure. — The  choroid  possesses  a  stroma  of  connective-tissue  with 
ramifying  corpuscles  containing  brown  or  black  pigment — melanin. 
This  stroma  is  lined  internally  by  a  structureless  layer — the  lamina 
vitrea,  and  it  supports  the  vessels  of  the  choroid.  The  arteries — which 
are  derived  from  the  ciliary  branches  of  the  ophthalmic — and  the  veins 
lie  together  in  the  outer  part  of  the  stroma,  while  the  capillaries  lie  in 
its  deeper  part  and  form  there  the  tunica  Ruyschiana.  The  smaller 
veins  converge  in  whorls — the  vasa  vorticosa — to  join  four  or  five 
principal  trunks.  The  ciliary  processes  have  the  same  structure  as  the 
choroid.  Each  contains  a  rich  plexus  of  tortuous  vessels.  The  branched 
cells  at  the  anterior  end  of  each  process  are  without  pigment.  Over  a 
considerable  area  on  the  inner  surface  of  the  choroid  the  pigment  is 
absent ;  and  there  the  choroid  shines  with  a  peculiar  iridescent,  metallic 
appearance  termed  the  tajMum  lucidum.  In  the  eyes  of  albinos  the 
choroid  is  entirely  free  from  pigment. 

The  Ciliary  Xerves  are  efferent  branches  of  the  lenticular  ganglion. 
They  perforate  the  sclerotic  in  company  with  the  ciliary  arteries,  and 
run  forwai'ds  between  the  sclerotic  and  cornea.  They  give  branches  to 
the  cornea  and  ciliary  muscle,  and  terminate  in  the  iris.  They  contain 
sensory  fibres,  which  are  derived  from  the  ophthalmic  division  of  the 
5th  nerve ;  motor  branches  to  the  ciliary  muscle  and  sphincter  muscle 
of  the  pupil,  which  come  from  the  third  nerve  ;  and  motor  fibres  to  the 


264  THE  ANATOMY  OF  THE  HORSE. 

dilator  muscle  of  the  pupil,  which  are  derived  from  the  sympathetic 
system. 

Directions. — In  the  immersed  eye  from  which  the  cornea  and  part  of 
the  sclerotic  have  been  removed,  the  portion  of  choroid  exposed  is  to  be 
torn  away  with  two  pairs  of  forceps  from  the  subjacent  retina.  The 
inner  surface  of  the  membrane  will  be  seen,  through  the  transparent 
vitreous  humoui*,  in  the  submerged  half  of  the  eye  that  was  frozen. 

The  Retina  is  the  most  delicate  of  the  coats  of  the  eyeball.  It  is 
foi'med  by  the  radiation  of  the  optic  nerve  on  the  inner  surface  of  the 
choroid,  and  like  that  coat  it  is  bell-shaped.  Its  external  or  choroidal 
surface  is  covered  by  a  layer  of  hexagonal  pigment  cells,  which  were  at 
one  time  referred  to  the  texture  of  the  choroid.  Its  inner  siu'face  is 
moulded  on  the  vitreous  humour.  This  surface  shows  a  little  to  the 
inner  side  of  the  summit  of  the  bell,  or  of  the  antero-posterior  axis  of 
the  eyeball,  a  disc-like  elevation — the  j^f^piHd  ojjtica,  which  is  the  point 
at  which  the  optic  nerve  begins  to  expand.  In  the  centre  of  this  spot 
the  arteria  centralis  retince  appears,  and  divides  into  branches  which 
radiate  on  the  inner  surface  of  the  retina.  The  nervous  structures  of 
the  retina  terminate  at  a  wavy  line — the  ora  serrata — behind  the 
ciliary  processes ;  but  the  retina  is  continued  beneath  these  processes 
in  the  form  of  an  epithelial  layer — the  j^cirs  ciliaris  retince,  which  forms 
the  edge  of  the  bell. 

In  the  human  03-6  a  yellow  spot — the  macula  httea — is  placed  a  little 
external  to  the  papilla  optica,  and  almost  exactly  in  the  antero-posterior 
axis  of  the  eyeball.  This  is  not  present  in  the  eye  of  the  horse  or  in  any 
mammal  lower  than  the  quadrumana. 

The  perfectly  fresh  retina  is  translucent  and  of  a  pale  pink  colour, 
but  it  speedily  becomes  opaque.  In  consistence  it  is  delicate  and 
jelly-like. 

Structure. — Ten  distinct  layers  are  described  as  composing  the  thick- 
ness of  the  i-etina.  These  enumerated  from  within  to  without  are  as 
follows : — 

The  Memhrana  Limitans  Interna. — This,  although  ajjpearing  as  a 
distinct  line  in  a  transverse  section,  is  not  a  distinct  stratum,  but  merely 
the  inner  limiting  line  of  a  sustentacular  framework — the  radial  fibres  of 
Miiller — which  pervades  and  supports  the  nervous  elements  in  the 
other  layers  of  the  retina. 

2.  The  Layer  of  Nerve  fibres. — This  layer  results  from  the  radiation 
of  the  optic  nerve,  whose  fibres  at  their  point  of  entrance  into  the  eye- 
ball lay  aside  their  medullar}^  sheath. 

3.  The  Layer  of  Nerve  Cells. — This  is  a  single  layer  of  multipolar 
nerve  cells. 

4.  The  Inner  Molecular  Layer  is  a  thick  stratum  of  fibres  and  inter- 
mediate granular  matter. 


DISSECTION    OP   THE   EYEBALL.  265 

5.  The  Inner  Nuclear  Layer  contains  spindle-shaped  or  bipolar  nerve 
cells  with  distinct  oval  nuclei  and  only  a  small  amount  of  protoplasm. 
The  inner  and  outer  poles  of  the  cells  are  continued  through  the  4th 
and  6th  layers  respectively. 

6.  The  Outer  Molecular  Layer  repeats  the  structure  of  the  inner 
molecular  layer. 

7.  The  Outer  Nuclear  Layer  contains  spindle-shaped  cells  with  con- 
spicuous nuclei  and  a  small  amount  of  protoplasm,  the  poles  of  the  cells 
being  prolonged  as  in  the  case  of  the  similar  elements  in  the  inner 
nuclear  layer. 

8.  The  Memhrana  Limitans  Externa. — This  is  the  outer  boundary  of 
the  sustentacular  framework  of  fibres  already  mentioned. 

9.  The  Layer  of  Rods  and  Cones,  or  the  hacillary  layer,  is  composed 
of  two  different  kinds  of  elements.  The  longer  elements,  the  rods, 
extend  vertically  between  the  8th  and  10th  layers;  the  cones  are  much 
shorter  than  the  rods,  and  do  not  reach  so  far  as  the  next  layer. 

10.  The  Pigmented  Epithelium. — This  is  a  layer  of  polygonal  pig- 
mented cells,  generally  six-sided. 

Directions. — The  third  eye  should  be  transversely  divided  with  a 
sharp  scalpel,  about  half  an  inch  behind  the  junction  of  the  sclerotic 
and  cornea.  This  should  be  done  with  the  eye  immersed  in  water. 
The  posterior  half,  after  removal  of  the  vitreous  humour,  shovild  be  used 
for  the  better  examination  of  the  inner  surface  of  the  retina.  The  lens 
should  be  removed  for  examination  from  the  anterior  half  In  the  eye 
already  used  for  the  display  of  the  retina,  that  coat  should  be  in  part 
removed,  so  as  to  display  the  vitreous  humour  with  the  lens  imbedded 
in  its  anterior  part.  By  a  combined  examination  of  all  the  pi'eparations, 
the  following  points  regarding  the  lens  and  vitreous  body  may  be  made 
out. 

The  Lens  is  situated  behind  the  pupil,  and  is  contained  within  a 
capsule  of  its  own. 

The  Capsule  is  a  close-fitting,  firm,  transparent  membrane,  which  is 
four  or  five  times  thicker  on  the  fi'ont  than  on  the  back  of  the  lens. 
The  anterior  surface  of  the  capsule  forms  the  posterior  boundary  of  the 
cavity  in  which  the  aqueous  humom-  is  contained,  and  the  iris  in  its 
movements  glides  on  it.  At  its  periphery  the  suspensory  ligament  of 
the  lens  blends  with  it.  The  posterior  surface  is  in  contact  with  the 
vitreous  humour. 

The  lens  is  a  transparent  solid  body  of  a  biconvex  shape,  the  convexity 
of  its  posterior  surface  being  considerably  greater  than  that  of '  the 
antei'ior.  It  is  maintained  in  a  depression  on  the  front  of  the  vitreous 
humour  by  a  suspensory  ligament.  This  ligament,  which  is  also  known 
as  the  zonula  of  Zinn,  arises  behind  and  beneath  the  ciliary  processes, 
where   it   is  connected  with   the   hyaloid  membrane   of  the   vitreous 


266  THE  ANATOMY  OF  THE  HORSE. 

humour.  It  passes  over  the  rim  of  the  lens,  and  blends  with  the 
anterior  part  of  the  lens-capsule.  Behind  the  rim  of  the  lens  the 
ciliary  processes  rest  on  the  outer  suxface  of  the  ligament ;  and  when 
these  are  removed,  the  ligament  is  there  seen  to  have  a  fluted  or  plaited 
appearance,  each  plait  fitting  into  the  depression  between  two  processes. 
At  this  same  point  the  inner  surface  of  the  zon\ila  forms  the  outer 
boundary  of  a  triangular  chink  which  runs  round  the  lens  behind  its 
rim.  This  is  the  canal  of  Petit,  which  is  bounded  in  front  by  the  lens- 
capsule,  behind  by  the  hyaloid  membrane  of  the  vitreous  humoiu-,  and 
outwardly  by  the  zonula. 

Structure. — When  removed  from  its  capsule,  the  lens  is  found  to  be 
soft  and  pulpy  in  its  outer  portion,  but  its  density  increases  in  passing 
from  the  sui-face  to  the  centre.  Both  its  surfaces  show  some  faint 
white  lines  radiating  from  the  central  point  of  the  surface.  The  number 
of  these  lines  varies  in  the  adult,  but  in  the  foetus  they  are  three  in 
number,  and  each  line  on  the  posterior  surface  is  in  position  midway 
between  two  of  the  anterior  lines. 

A  lens  that  has  been  hardened  in  spirit  or  by  boiling  may  be  broken 
down  into  concentric  laminae  like  the  coats  of  an  onion.  Each  of  these 
laminae  is  composed  of  long  riband-shaped  fibres.  These  lens-jihres  when 
examined  microscopically  are  seen  to  have  finely  serrated  edges  by 
which  adjacent  fibres  are  interlocked. 

The  foetal  lens  is  nearly  spherical,  it  is  of  a  reddish  colour,  and  not 
quite  transparent.  In  the  young  adult  it  is  distinctly  biconvex,  firm, 
colourless,  and  transparent.  With  advancing  age  it  tends  to  become 
flatter,  denser,  less  transparent,  and  of  a  yellowish  colour. 

The  Vitreous  Humour  occupies  four-fifths  of  the  interior  of  the  eye- 
ball. It  is  globular  in  form,  with  a  depression  in  front  for  the  lodgment 
of  the  lens.  It  is  colourless,  transparent,  and  of  a  consistency  like  thin 
jelly.  It  is  enveloped  by  a  delicate  capsule — the  hyaloid  membrane,  which 
is  connected  in  front  with  the  suspensory  ligament  of  the  lens,  and  ends 
by  joining  the  capsule  behind  the  lens.     . 

Striicttire. — The  vitreous  humour  is  composed  of  branched  connective- 
tissue  corpuscles  in  a  jelly-like  matrix. 


CHAPTER    VIII. 


THE    EAR. 

The  organ  of  hearing  consists  of  three  divisions — the  external,  the 
middle,  and  the  internal  ear.  The  first  of  these  comprises  the  osseous 
external  auditory  process,  and  the  trumpet-like  organ  Avhich  collects  the 
waves  of  sound  and  transmits  them  along  that  process  to  the  middle 
ear.     It  is  described  at  page  159. 

The  middle  and  the  internal  ear  are  cavities  excavated  in  the  sub- 
stance of  the  petrous  tempoi-al  bone.  From  their  situation  and  the 
minuteness  and  intricacy  of  their  parts,  their  dissection  is  extremely 
difficult.  The  student  is  therefore  recommended  to  study  the  anatomy 
of  these  parts  on  the  models  and  special  dissections  to  which  he  is 
likely  to  have  access,  and  by  the  aid  of  the  fuller  description  given  in 
systematic  text-books.  At  the  same  time,  an  outline  description  will 
be  here  given,  which  the  student  may  illustrate  to  himself  by  procuring 
two  or  three  petrous  temporal  bones  and  dissecting  them  after  they 
have  been  decalcified  in  a  hydrochloi-ic  or  chromic  acid  solution. 

THE    MIDDLE    EAR. 

The  Middle  Ear — called  also  the  Tympanum,  or  drum  of  the  ear — 
is  a  cavity  of  the  petrous  temporal  bone.  It  contains  air,  and 
across  it  there  stretches  a  chain  of  minute  bones,  which  transmit  the 
sound  waves  from  the  outer  to  the  inner  ear.  The  inner  wall  of  the 
chamber  is  fonned  by  that  portion  of  bone  in  which  the  divisions  of  the 
internal  ear  are  excavated,  and  it  shows  the  following  objects  : — The 
2)rom<mtory — a  projection,  or  bulging,  which  con-esponds  to  the  first  turn 
of  the  cochlea.  Above  the  promontory,  the  fenestra  ovalis — an  opening 
which  is  closed  by  the  base  of  the  stapes  (the  innennost  of  the  auditory 
ossicles).  Below  the  promontory,  another  opening — the  fenestra  rotunda, 
which  is  closed  by  a  thin  membrane.  A  pin  passed  through  the 
fenestra  ovalis,  would  enter  the  vestibular  division  of  the  internal  ear ; 
if  passed  through  the  fenestra  rotunda,  it  would  peneti'ate  the  scala 
tympani  of  the  cochlea.  The  outer  wall  of  the  chamber  is  formed 
mainly  by  the  memhrana  tympani.  This  is  a  thin,  translucent  membrane 
which  forms  the  septum  between  the  tympanum  and  the  outer  ear. 


268  THE  ANATOMY  OF  THE  HORSE. 

The  rim  of  the  membrane  is  fixed  in  a  groove  of  the  bone.  The  mem- 
brane is  slightly  cupped  towards  the  outer  ear ;  while  its  inner  surface 
is  convex,  and  has  the  handle  of  the  malleus  (the  outermost  ossicle) 
attached  to  it.  The  surfaces  of  the  membrane  are  inclined  so  that  the 
outer  surface  looks  somewhat  downwards,  and  the  inner  upwards.  In 
structure  the  membrane  comprises  (1)  a  middle  fibrous  stratum,  the 
fibres  being  arranged  both  radially  and  circularly,  with  (2)  an  outer 
and  (3)  an  inner  epithelial  covering.  The  roof  and  the  floor  of  the 
tympanum  present  nothing  of  interest.  The  former  is  the  more  exten- 
sive. The  anterior  extremity  of  the  chamber  shows  a  fissure  by  which 
air  is  admitted  from  the  Eustachian  tube.  Through  this  opening  also 
the  mucous  lining  of  the  cavity  is  continuous  with  that  of  the  Eustachian 
tube.  The  posterior  extremity,  and  part  of  the  floor  and  outer  wall 
communicate  with  the  cellular  spaces  of  the  mastoid  protuberance. 

The  Auditory  Ossicles. — There  are  three  of  these,  viz.,  the  malleus, 
the  inciis,  and  the  stapes. 

The  Malleus,  named  from  its  resemblance  to  a  hammer,  is  the 
largest  bone.  It  possesses  a  head,  a  handle,  and  two  j)r<^cesses.  The 
head  is  articulated  by  a  synovial  joint  to  the  stapes.  The  handle  is 
fixed  on  the  inner  surface  of  the  membrana  tympani.  The  lony  jjrocess 
is  slender,  and  projects  forwards  to  be  fixed  in  a  slit  of  the  petrous 
temporal.  The  short  process  is  a  mere  projection  of  the  root  of  the 
handle,  and  is  fixed  to  the  membrana  tympani. 

The  Incus  is  named  from  its  supposed  resemblance  to  an  anvil,  but  it 
has  more  likeness  to  a  human  bicuspid  tooth.  It  presents  a  body  and 
two  2^rocesses,  or  crura.  The  body  has  a  saddle-shaped  articular  facet  for 
the  malleus.  The  short  p7-ocess  is  directed  backwards  to  be  fixed  to  the 
wall  of  the  tympanum.  The  ^o?iy^j);-oef«5  curves  downwards  and  inwards 
to  terminate  in  a  rounded  point — the  orbicular  jyrocess,  which  articulates 
with  the  head  of  the  stapes. 

The  Stapes  is  stirrup-shaped.  It  is  the  smallest  bone,  and  possesses 
a  head,  a  neck,  a  base,  and  two  crura.  The  head  is  depressed  for  articula- 
tion with  the  orbicular  process,  and  is  succeeded  by  the  slightly 
constricted  necl:  The  base  is  a  thin  plate  which  closes  the  fenestra 
ovalis.  The  crura  are  slender  rods  of  bone  connecting  the  base  and  the 
neck. 

Muscles  of  the  Ossicles. — These  are  two — the  tensor  tympani  and  the 
stapedius.  (The  so-called  laxator  tympani  is  now  believed  to  be  a 
ligament.) 

The  Tensor  Tympani  arises  from  the  petrous  temporal  bone  near  the 
Eustachian  orifice,  and  it  is  inserted  by  a  slender  tendon  into  the  handle 
of  the  malleus  near  its  root. 

Action.  —To  tense  the  membrana  tympani. 

The  Stapedius  arises  within  the  pyrcnnid — a  small  process  of  bone  at 


THE    BAR.  269 

the  back  of  the  tjrmpanum.  Issuing  from  the  pyi'amid,  it  is  inserted 
into  the  neck  of  the  stapes.  Its  tendon  of  insertion  contains  a  smaU 
nucleus  of  bone. 

Action. — To  regulate  (diminish  the  excursions  of)  the  movements  of 
the  stapes. 

Bloodvessels.  The  arteries  of  the  tympanum  are  derived  from  the 
tympanic  artery,  a  branch  of  the  internal  maxillary  artery. 

Nerves.  The  choi-da  tympani  branch  of  the  7th  nerve  enters  the 
cavity  of  the  tympanum  from  the  aqueduct  of  Fallopius;  and  passing 
across  the  membrana  tympani  it  leaves  the  cavity  by  the  styloid  fora- 
men. The  sensory  nerves  of  the  tympanum  are  derived  from  the 
tympanic  branch  ( Jacobson's  nerve)  of  the  glosso-pharyngeal. 

The  Nerve  to  the  Staj)eclitis  is  a  branch  of  the  7th. 

The  Nei-ve  to  the  Tensor  Tympani  comes  from  the  5th,  through  the 
otic  ganglion. 

THE   INTERNAL    EAR. 

The  Internal  Ear,  called  also,  from  its  complexity,  the  Labyrinth, 
consists  of  a  series  of  chambers,  or  passages,  in  the  petrous  temporal 
bone,  and  of  certain  fluids  and  soft 
textures  contained  within  these 
passages.  The  chambers,  with  the 
wall  of  condensed  bone  tissue  which 
immediately  surrounds  them,  con- 
stitute the  osseous  labyrinth ;  the 
contained  soft  structures  form  the 
memh'anous  labyrinth.  The  osseous 
labyrinth  consists  of  three  divisions; 
— the  vestibule,  the  cochlea,  and  the 
semicircular  canals,  and  each  of  these  ^^^'  ^*' 

contains     a     division    of     the     mem-    Diagram  of  the  membranous  Labyrinth. 
K,.o,-./Mnc!  lal-.Tr>-ii-.+lT  DC.  Ductiis  cochlearis  ;  dr.  Ductus  reuniens  ; 

bianOUS  labyimtla.  S.  Sacculus;  U.   Utriculus;     dv.  Ductus  vesti- 

The  Vestibule.  This  is  the  ]^a}ij,^)  ^^°'^"'^'=''^^''  °*°^^'-  (^«'-»»«-. »"« 
central  division  of  the  labyrinth.    It 

lies  between  the  inner  wall  of  the  tympanum  and  the  internal  audi- 
tory meatus.  In  front  it  communicates  with  the  scala  vestibuli  of  the 
cochlea,  and  the  semicircular  canals  open  into  it  behind  by  five 
openings.  On  its  outer  wall,  w^hich  separates  it  from  the  tym- 
panum, is  the  fenestra  ovalis,  closed  by  the  base  of  the  stapes.  On 
its  inner  wall  in  front  there  is  a  depression — the  fovea  hemisjoherica — 
placed  over  the  meatus  auditorius  internus,  and  pierced  by  minute 
foramina  for  the  passage  of  the  filaments  of  the  auditory  nerve. 
Behind  the  fovea  hemispherica  is  a  small  slit  which  leads  into  the 
aqueductus  vestibuli.  The  roof  of  the  vestibule  shows  another  depres- 
sion— the  fovea  hemi-ellij)tica. 


270  THE  ANATOMY  OF  THE  HORSE. 

Contained  immediately  within  the  osseous  vestibule  there  is  a  quantity 
of  limpid,  serous  fluid — the  perilymph,  which  surrounds  the  parts  of  the 
membranous  labyrinth  here  found.  These  are  two  delicate  sacs — the 
saccule  and  the  utricle. 

The  Saccule  is  the  antei'ior  and  smaller  of  the  two  sacs,  and  is  lodged 
in  the  fovea  hemispherica.  It  contains  a  fluid  termed  the  endolymph. 
It  communicates  with  the  membranous  canal  of  the  cochlea  by  a 
minute  tube — the  canalis  7-euniens,  and  with  the  utricle  by  a  Y  shaped 
tube — the  diictus  vestibuli,  the  stem  of  which  ends  blindly  in  the  aqueduct 
of  the  vestibule. 

The  Utricle,  placed  above  and  behind  the  saccule,  is  lodged  in  the 
fovea  hemi-elliptica.  Like  the  saccule,  it  contains  endolymph.  It  com- 
municates, as  aforesaid,  with  the  saccule ;  and  the  five  openings  of  the 
membranous  semicircular  canals  open  directly  into  it. 

The  interior  of  both  saccule  and  utricle  is  elevated  into  a  ridge — the 
crista  aconstica,  in  which  are  distributed  the  terminal  filaments  of  the 
vestibular  division  of  the  auditory  nerve.  On  this  crest  are  certain 
peculiar  cells,  each  having  a  peripheral  hair-like  process  which  projects 
into  the  endolymph,  and  a  central  process  which  is  probably  continuous 
with  a  filament  of  the  auditory  nerve.  Here  are  also  found  the  otoliths, 
which  are  minute  calcareous  particles  imbedded  in  a  jelly-like  material. 
The  Semicircular  Canals  are  placed  behind  the  vestibule.  They  are 
three  in  number  and  are  distinguished  as  superior,  posterior,  and  external. 
The  two  first  have  a  vertical  direction,  while  the  latter  is  nearly  hori- 
zontal. Each  canal  opens  into  the  vestibule  by  a  dilated  extremity, 
termed  the  ampulla.  The  non-ampullated  end  of  the  external  canal 
opens  by  an  independent  orifice  into  the  vestibule,  while  the  non-ampull- 
ated ends  of  the  other  two  canals  have  a  common  opening  into  the  same 
cavity.  The  three  canals  have  thus  five  openings  into  the  vestibule, 
and  three  of  these  openings  are  ampullated. 

The  Membranous  Semicircular  Canals. — Contained  immediately  within 
the  osseous  canals  is  a  quantity  of  penlymph,  which  surrounds  the 
membranous  canals.  Each  of  these  repeats  the  form  of  the  osseous 
canal  in  which  it  is  lodged  ;  and  they  communicate  with  the  utricle  by 
five  openings,  three  of  which  are  ampullated.  The  membranous  canals 
contain  endolymph,  and  the  ampullated  end  of  each  is  raised  inwardly 
into  a  ridge,  or  acoustic  crest,  having  hair  cells,  otoliths,  and  nerve 
terminations  similar  to  those  of  the  saccule  and  utricle. 

The  Cochlea  is  named  from  its  resemblance  to  a  snail's  shell.  It  has 
the  form  of  a  slightly  tapering  tube  wound  spirally  two  and  a  half 
times  around  a  central  axis — the  modiolus.  It  is  thus  somewhat 
conical  in  form,  the  base  lying  inwards  near  the  internal  auditory 
meatus,  from  whicli  point  the  axis  of  the  cone  is  directed  outwards,  for- 
wards, and  downwards  to  the  apex.     Projecting  half  way  into  the  tube 


THE    EAR.  271 

of  the  cochlea  is  a  lamina,  or  shelf,  of  bone — tei'med  the  osseous  spiral 
lamina.  The  tube  is  thus  imperfectly  divided  into  two  passages,  termed 
respectively  the  scala  timpani  and  the  scala  vestibuli.  The  separation 
between  these  two  passages  is  rendered  moi-e  complete,  and  a  third 
passage  is  marked  off,  by  certain  membranous  structures.  These  are 
the  basilar  membr^ane  and  Beissner's  membrane.  The  basilar  membrane 
stretches  from  the  free  edge  of  the  osseous  spiral  lamina  to  the  outer 
wall  of  the  tube,  where  it  joins  a  thickening  of  the  lining  of  the  tube, 
termed  the  Sjyiral  ligament.  Reissnerh  Membrane  is  much  more  delicate, 
and  stretches  from  the  adsta  spiralis  at  the  free  edge  of  the  osseous 
spiral  lamina,  obliquely  upwards  and  outwards  to  the  wall  of  the  tube. 


Fig.  35. 
Transverse  Section  through  the  Tube  of  the  Cochlea. 
ill.  Modiolus  ;   O.  Outer  wall  of  cochlea ;  SV.  Scala  vestibuli ;   ST.  Scala  tympani ;  DC.  Ductus 
cochlearis ;  »)iR.  Membrane  of  Reissner  ;  bm.  Basilar  membrane ;  sc.  Crista  spiralis ;  si.  Spiral  liga- 
ment ;  sg.  Spiral  ganglion  of  auditory  nerve  ;  oc.  Organ  of  Corti  {Turner). 

The  tube  is  thus  divided  into  three  passages,  viz. ,  the  scala  tympani, 
the  scala  vestibuli,  and  the  scala  intermedia. 

The  Scala  Tympani  is  the  largest  of  the  three  passages,  and  is 
separated  from  the  other  two  by  the  osseous  spiral  lamina  and  the 
basilar  membrane.  At  the  base  of  the  cochlea  it  begins  at  the  fenestra 
rotunda,    by   which,    in   the   dried    bone,    it   communicates   Avith  the 


272  THE  ANATOMY  OF  THE  HORSE. 

tympanum.  At  the  apex  of  the  cochlea  it  communicates  with  the  scala 
vestibuli  by  a  small  opening — the  helicotrema. 

The  Scala  Vestibuli  is  separated  from  the  preceding  by  the  osseous 
spiral  lamina,  and  from  the  scala  intermedia  by  Reissner's  membrane. 
At  the  apex  of  the  cochlea  it  communicates  with  the  scala  tympaui  by 
the  helicotrema,  and  at  the  base  it  opens  freely  into  the  osseous  vestibule. 
Like  the  vestibule,  it  therefore  contains  perilymph,  and  this  passes  also 
by  the  helicoti-ema  into  the  scala  tympani. 

The  Scala  Intermedia  is  the  smallest  but  the  most  important  of  the 
three  passages.  It  is  the  true  viemhranous  cochlea,  and  is  called  also  the 
ductus  cochlearis.  It  is  separated  from  the  scala  vestibuli  by  the  mem- 
brane of  Reissner ;  and  from  the  scala  tympani  mainly  by  the  basilar 
membrane,  but  partly  by  the  osseous  spiral  lamina  near  its  free  edge. 
At  the  base  of  the  cochlea  it  communicates  by  the  slender  canalis 
reuniens  with  the  sacculus,  and  it  thus  contains  endolymph. 

The  terminal  filaments  of  the  cochlear  division  of  the  auditory  nerve 
are  distributed  in  the  substance  of  the  basilar  membrane  ;  and  on  that 
surface  of  the  membrane  which  is  directed  towards  the  scala  intermedia, 
there  occurs  a  peculiar  arrangement  of  cells,  termed  the  orga7i  of  Corti. 

The  Organ  of  Corti. — When  the  basilar  membrane  is  examined  in 
transverse  section,  it  is  seen  to  support  about  the  centre  of  the  surface 
directed  towards  the  scala  intermedia  a  double  row  of  elongated  rod- 
like cells,  termed  Corti's  rods.  The  rods  of  the  two  rows,  where  they 
rest  on  the  basilar  membrane,  are  separated  by  a  slight  interval ;  but 
they  incline  towards  each  other  and  meet  at  the  opposite  extremity,  so 
as  to  enclose  a  minute  canal — the  caned  of  Corti.  On  the  outer  side 
of  the  external  row  of  rods,  the  basilar  membrane  supports  four  or  five 
rows  of  shorter  cells,  the  free  extremity  of  each  of  which  bears  a  tuft  of 
stiff,  hair-like  processes.  In  the  same  way  the  membrane  supports  a 
single  row  of  hair-bearing  cells  on  the  inner  side  of  the  inner  rods  of 
Corti.  On  either  side  these  hair -hearing  cells  are  succeeded  by  cells 
which  become  progressively  shorter  and  pass  into  the  general  columnar 
cell  lining  of  the  scala  intermedia.  A  delicate  cellular  membrane — the 
membrana  reticularis — is  spread  over  the  outer  hair-bearing  cells. 
Through  apertm-es  in  this  membrane,  the  tufts  of  hair-like  processes 
project,  in  a  manner  comparable  to  tufts  of  grass  springing  through  the 
interstices  of  a  wire  net.  Still  another  membrane — the  membrana  tectoria 
— springs  from  the  edge  of  the  osseous  spiral  lamina  between  the  lines  of 
origin  of  the  basilar  and  Reissner's  membranes,  and  passes  outwards 
over  the  organ  of  Corti. 

The  Auditory  (8th)  Cranial  Nerve.  This  nerve  enters  the  inter- 
nal auditory  meatus  in  company  with  the  7th,  which  passes  into  the 
aqueduct  of  Fallopius.  The  8th  divides  into  two  branches,  one  for  the 
cochlea,   the   other   for   the   vestibule    and   semicircular   canals.      The 


THE    EAE.  273 

filaments  of  the  latter  branch  penetrate  the  minute  foramina  seen  at  the 
bottom  of  the  internal  auditory  meatus,  and  are  finally  distributed  in 
the  saccule,  utricle,  and  ampullated  ends  of  the  membranous  semicircular 
canals.  The  cochlear  branch  penetrates  the  modiolus,  and  in  its  passage 
detaches  twigs  which  pass  outwards  in  the  osseous  spiral  lamina  to  reach 
the  basilar  membrane.  Within  the  spiral  lamina  there  are  numerous 
ganglion  cells  placed  on  the  course  of  the  nerve  fibres. 


CHAPTER    IX. 

DISSECTION   OF   THE   PERIN.EUM   IN   THE  MALE.* 

Under  this  section  there  will  be  described  not  only  the  perinteum 
proper,  but  also  the  scrotum,  testicle,  prepuce,  and  penis.  The  dissec- 
tion of  all  these  must  precede  that  of  the  hind  limb  and  abdomen,  and 
it  should  therefore  be  begun  without  delay. 

THE    PERINiEUM. 

Position. — Place  the  animal  on  the  middle  line  of  its  back,  and  draw 
its  hind  legs  upwards  and  outwards  by  ropes  running  over  pulleys  fixed 
to  the  ceiling.  The  posterior  extremity  of  the  trunk  should  be  level 
with,  or  project  slightly  over,  the  end  of  the  table  on  which  the  subject 
rests.  Empty  the  posterior  pai't  of  the  rectum,  and  stviff  it  with  tow 
saturated  in  some  preservative  solution.  A  stitch  should  then  be  put 
through  the  edges  of  the  anus. 

Surface-mai-king. — The  deep  boundaries  of  the  perinajum  are  those  of 
the  outlet  of  the  pelvis  (page  341),  but  its  superficial  boundaries  are  as 
follows : — Above  it  is  limited  by  the  root  of  the  tail,  on  each  side  it  is 
bounded  by  the  semimembranpsus  muscle,  and  inferiorly  it  is  continued 
without  any  limit  into  the  cleft  between  the  thighs. 

On  the  middle  line  below  the  root  of  the  tail  is  the  anus.  This  forms 
an  eminence  more  pronounced  in  the  young,  than  in  the  old,  animal. 
The  integumental  covering  of  the  eminence  is  thin,  puckered,  and  hair- 
less ;  and  it  is  generally  dark-pigmented.  Passing  between  the  rectum 
and  the  root  of  the  tail  on  each  side,  and  most  distinct  when  the  latter 
is  forcibly  elevated,  there  is  a  projection  caused  by  the  so-called  suspen- 
sory ligament  of  the  rectum.  Beneath  the  anus  there  can  be  seen  or 
felt  a  longitudinal  prominence  formed  by  the  urethra;  and  on  the  middle 
line  of  this,  there  is  a  median  raphe  which  is  prolonged  between  the 
thighs. 

Directions. — Make  a  mesial  incision  through  the  skin  for  a  length  of 
six  inches  below  the  anus.  Carry  this  incision  round  the  sides  of  the 
anus,  and  up  to  the  root  of  the  tail.  Make  another  incision  trans- 
versely from  one  tuber  ischii  to  the  other.     These  incisions  will  enable 

*  The  description  of  the  peiinajum  in  the  female  is  incorporated  with  that  of  the  pelvis. 


DISSECTION    OF    THE    PERINEUM    IN    THE    MALE.  275 

sufficient  skin  to  be  raised  as  fom*  triaugulai-  flaps.  Around  the  anus 
there  is  a  quantity  of  fat,  whose  amount  varies  with  the  condition  of  the 
subject,  but  is  greater  in  the  young,  than  in  the  old,  animal.  In  this 
fat  the  perineal  nerves  are  to  be  followed. 

Perineal  Cutaneous  Nerves. — 1.  Hoemorrhoidal  Branch  of  5th  Sacral 
Nerve.  This  nerve  will  be  found  emerging  at  the  hinder  edge  of  the 
coccygeal  origin  of  the  semimembranosus,  and  curving  downw'ards  and 
inwards  at  the  root  of  the  tail.  It  supplies  the  skin  there,  and  gives 
some  twigs  dowiiwards  to  the  skin  of  the  anus. 

2.  Hoemorrhoklal  Nerve.  The  trunk  of  the  hsemorrhoidal  nerve, 
which  cannot  be  reached  at  pi'esent  (page  343),  divides  between  the 
sacro-sciatic  ligament  and  the  retractor  ani.  Its  branches  are  as 
follows  : — 1.  A  branch  appears  at  the  inner  side  of  the  coccygeal  origin 
of  the  semimembranosus,  and  is  distributed  at  the  side  of  the  anus.  2. 
External  to  the  preceding  a  branch  perforates  the  semimembranosus ;  and 
descending  over  the  tuber  ischii,  it  is  distributed  at  the  side  of  the 
penis.  3.  About  an  inch  or  two  below  the  anus  a  branch  appears  near 
the  middle  line,  and  descends  over  the  urethra. 

3.  Puclic  Nerve.  Ascending  on  the  side  of  the  anus,  beneath  the 
branches  of  the  h^emorrhoidal  nerve,  are  some  twigs  from  the  pudic 
nerve.     They  terminate  in  the  skin  and  the  sphincter  ani. 

Perineal  Fascia.  The  lower  part  of  the  perinseum  is  covered  by  two 
layers  of  fascia,  viz.,  a  superficial  and  a  deep.  The  superficial  layer  is 
attached  laterall}-  to  the  fascia  covering  the  muscles  on  the  inside  of  the 
thigh,  towards  the  anus  it  loses  its  aponeurotic  character  and  becomes 
cellular,  and  inferiorly  it  blends  -with  the  dartos.  The  deep  layer  is 
reflected  upwards  at  each  side  of  the  penis,  while  above  and  below  it 
loses  its  distinctness  and  becomes  cellular. 

Directions. — These  layers  of  fascia  should  be  removed,  and  the  parts 
should  be  cleaned  after  the  manner  of  Plate  37.  Beneath  the  deep 
layer  a  branch  of  the  pudic  nerve  will  be  found  descending  on  the  acceler- 
ator urinre  muscle.  The  transversus  perineei,  if  present  (it  was  absent 
in  the  subject  from  which  the  Plate  was  taken),  will  be  found  concealing 
the  internal  pudic  artery,  and  may  be  removed  on  one  side. 

The  Internal  Pudic  Artery.  This  vessel  is  a  branch  of  the  internal 
iliac  artery  (Plates  46  and  47).  It  descends  obliquely  along  the  side  of 
the  pelvis,  on  the  inner  side  of  the  sacro-sciatic  ligament  or  within  its 
texture.  At  the  small  sacro-sciatic  foramen  it  passes  backwards  and 
inwards  to  turn  round  the  ischial  arch.  It  penetrates  the  m-ethral  bulb, 
immediately  resolving  itself  into  a  number  of  branches  that  supply  the 
erectile  tissue  of  that  body.  Its  position  should  be  particularly  noted 
with  reference  to  the  operation  of  lithotomy,  in  which,  by  making  a  mesial 
incision,  the  urethra  may  be  opened  without  danger  of  wounding  the 
artery. 


276  THE  ANATOMY  OF  THE  HORSE. 

In  this  part  of  its  course  the  vessel  gives  off  small  branches  to  the 
anus  and  to  the  erector  penis  muscle. 

The  Internal  Pudic  Vein  accompanies  the  artery. 

The  Sphincter  Ani  Externus.  The  fibres  of  this  muscle  are  of  the 
striped  variety,  and  they  are  circularly  disposed  around  the  anus. 
Above  the  anus  the  fibres  are  fixed  at  the  root  of  the  tail,  and  below  it 
they  unite  to  form  a  pointed  slip  inserted  into  the  perineal  fascia.  The 
muscle  should  be  removed  in  order  to  expose  the  next. 

The  Sphincter  Ani  Internus.  This  is  comprised  between  the  outer 
muscle  and  the  mucous  membrane.  Its  fibres  are  circularly  disposed 
like  those  of  the  external  sphincter,  from  which  they  differ  in  being  of 
the  non-striped  variety.  They  are,  in  fact,  nothing  more  than  the  last 
of  the  circular  muscular  fibres  of  the  rectum ;  and  in  the  horse  they  are 
not  aggregated  in  the  form  of  a  ring,  as  they  are  in  man. 

Action  of  the  sphincters. — To  maintain  the  anus  closed  except  during 
the  passage  of  excreta. 

The  Retractor  Ani  (Levator  ani  of  human  anatomy).  This  nmscle  is 
red  like  the  external  sphincter.  It  arises  (but  this  cannot  be  seen  at 
present)  from  the  superior  ischiatic  spine,  and  from  the  inner  sm-face  of 
the  sacro-sciatic  ligament  over  the  small  sacro-sciatic  foramen.  Its 
fibres  pass  upwards  and  backwards,  and  terminate  in  tendinous  slips 
that  are  insinuated  beneath  the  anterior  edge  of  the  external  sphincter. 

Action. — During  the  passage  of  fasces  the  anus  is  carried  backwards 
and  everted,  and  the  action  of  this  m\iscle  is  to  carry  the  anus  forwards 
and  invert  it  after  the  act  of  defsecation. 

The  Retractor  Penis.  This  muscle  descends  at  the  side  of  the  rec- 
tum, immediately  in  front  of  the  external  sphincter,  and  under  cover  of 
the  termination  of  the  retractor  ani,  which  must  therefore  be  raised  and 
turned  forwards.  The  fibres  of  the  muscle  are  non-striped,  and  they 
form  a  narrow  riband  which  arises  from  the  1st  and  2nd  or  2nd  and 
3rd  coccygeal  bones.  The  right  and  left  bands  meet  below  the  rectum, 
for  which  they  thus  form  a  kind  of  sling.  They  arc  then  prolonged 
downwards  on  the  middle  line  of  the  corpus  spongiosum,  on  which  they 
are  lost  near  the  extremity  of  the  penis. 

Action. — To  i-etract  the  penis  within  the  prepuce  when  erection  passes 
off". 

The  Suspensory  Ligament  of  the  Rectum  (Plate  46).  This,  although 
denominated  a  ligament,  is  composed  of  non-striped  muscular  tissue.  It 
is  derived  from  the  longitudinal  muscular  fibres  of  the  rectum,  which  it 
leaves  in  front  of  the  external  sphincter;  and  passing  upwards,  it 
becomes  inserted  into  the  4th  and  5th  coccygeal  vertebrae.  It  forms 
at  the  root  of  the  tail  a  prominence  which  has  already  been  referred  to. 

The  Transversus  Perin^i.  This  muscle  is  not  constantly  present. 
It  arises  from  the  tuber  ischii,  behind  the  origin  of  the  erector  penis ; 


DISSECTION    OF    THE    PERINEUM    IN    THE    MALE.  277 

and  it  passes  transversely  inwards  to  terminate  on  the  middle  line  over 
the  urethra,  being  confounded  with  its  fellow  of  the  opposite  side,  and 
with  the  first  fibres  of  the  accelerator  urinse. 

Action. — To  dilate  the  bulbous  part  of  the  urethra. 

The  Accelerator  UriNyE.  These  muscles  (right  and  left)  cover  the 
sides  and  lower  face  of  the  urethra  from  the  ischial  arch  to  the  free 
extremity  of  the  penis.  Along  the  inferior  median  line  of  that  tube  the 
right  and  left  muscles  are  joined  by  an  intermediate  fibrous  raphe. 
From  this  raphe  the  fibres  pass  round  the  urethra  on  each  side,  with  a 
slightly  forward  inclination,  and  are  lost  on  the  upper  aspect  of 
the  tube,  but  without  reaching  the  middle  line.  Beneath  the  anus  the 
first  fibres  of  the  muscle  seem  to  arise  from  the  retractor  penis  muscle, 
but  elsewhere  the  retractor  is  superficial  to  the  intermediate  raphe  of 
the  right  and  left  muscles. 

Action. — The  muscles  of  opposite  sides  always  act  together  ;  and  when 
they  do  so,  they  diminish  the  calibre  of  the  urethra  and  expel  its  con- 
tents. In  this  way  they  are  instrumental  in  the  ejaculation  of  semen. 
In  micturition  the  muscles  ordinarily  do  not  come  into  play  until  the 
close  of  the  act,  when  they  empty  the  urethra  from  behind  to  before. 
The  necessity  for  this  action  exists  because  the  expelling  power  of  the 
bladder  is  lost  as  soon  as  its  own  cavity  is  emptied. 

The  Erector  Penis.  This  is  a  thick,  dark-red  muscle  covering  the 
crus  penis.  Its  fibres  arise  from  the  inferior  ischiatic  spine  (of  the 
tuber),  and  they  terminate  on  the  crus. 

Action. — To  aid  in  erecting  the  penis  by  compressing  the  crus  and 
thus  retai-ding  the  return  of  blood  from  the  cavernous  body  of  the 
penis. 

the  scrotum  (plate  37). 

Position. — Let  the  subject  remain  in  the  dorsal  position,  but  unfasten 
the  rope  from  one  of  the  hind  limbs,  and  allow  the  trunk  to  incline  to 
the  same  side.  The  loose  limb  should  be  fastened  backwards  out  of  the 
way. 

Directions. — Grasp  the  neck  of  the  scrotum  close  to  the  wall  of  the 
abdomen,  so  as  to  tighten  the  skin  over  the  testicle,  and  then  tie  a  piece 
of  soft  cord  round  the  constricted  neck  of  the  scrotum.  This  will  facili- 
tate the  dissection  of  the  different  layers  of  the  pouch. 

The  scrotum  is  the  bag,  or  pouch,  in  which  the  testicles  are  sus- 
pended.    It  is  laminated,  and  comprises  the  following  layers  : — 

1.  The  Scrotal  Integument.  This  is  continuous  with  the  surround- 
ing skin,  of  which  it  is  a  modified  portion.  It  is  thin,  with  short  fine 
hairs,  and  numerous  sebaceous  glands,  whose  secretion  renders  it  moist. 
It  is  traversed  mesially  by  a  raphe,  continuous  posteriorly  with  the 
median  raphe  of  the  perinseum.  The  scrotal  integument  forms  a  single 
bag  for  the  two  testicles. 


2tii  THE   ANATOMY    OP    THE    HORSE. 

2.  The  Dartos.  If  a  portion  of  skin  be  removed  from  the  scrotum,  it 
will  expose  a  reddish-yellow  layer,  composed  of  connective-tissue  with 
many  elastic  fibres  and  a  considerable  quantity  of  involuntary  muscular 
tissue.  This  is  the  dartos,  and,  like  the  remaining  tunics  of  the 
testicle,  it  forms  two  distinct  pouches,  one  for  each  testicle.  In  the  mesial 
plane,  over  the  median  raphe,  the  right  and  left  pouches  are  applied 
together  and  form  the  septum  scroti;  but  superiorly  they  separate  to 
allow  the  penis  to  pass  between  them.  Traced  upwards,  the  dartos  is 
continuous  around  the  external  abdominal  ring  Avith  the  subcutaneous 
fascia.  Under  the  contraction  of  the  muscular  tissue  of  the  dartos,  the 
scrotum  becomes  firm  and  wrinkled;  during  relaxation  the  scrotum  is 
smooth  and  pendulous. 

3.  The  Spermatic  Fascia,  continuous  with  the  tendon  of  the  external 
oblique  tendon. 

4.  The  Cremasteric  Fascia,  continuous  with  the  internal  oblique 
muscle. 

5.  The  Infundibuliform  Fascia,  continuous  with  the  transversalis 
fascia. 

6.  The  Tunica  Vaginalis  Reflexa,  a  layer  of  serous  membrane  con- 
tinuous with  the  peritoneum  of  the  abdominal  cavity. 

In  Plate  37  these  layers  are  semi-diagrammatically  represented  as  suc- 
ceeding each  other  like  the  coats  of  an  onion.  The  dissector  will  probably 
be  unable  to  discriminate  layers  3,  4,  and  5.  The  tunica  vaginalis  reflexa, 
he  will  recognise  as  a  semitransparent  layer  which,  when  cut  through, 
takes  him  into  a  smoothly  lined  pouch  in  which  the  testicle  lies  free. 
This  is  the  sac  of  the  tunica  vaginalis,  a  diverticulum  of  the  peritoneal 
cavity. 

In  the  foetus  the  testicles  make  their  first  appearance  in  the  sub- 
lumbar  region,  close  behind  the  kidneys.  As  development  proceeds, 
they  descend  through  the  abdominal  wall  into  the  scrotiuu ;  and  hence 
the  correspondence  between  the  coverings  of  the  testicle  and  the  layers 
that  compose  the  wall  of  the  abdomen. 

If  the  dissector  will  now  lay  hold  of  the  testicle,  and  endeavour  to 
drag  it  out  of  the  opening  which  he  has  made  in  its  coverings,  he  will 
bring  into  view  the  spermatic  cord.  The  testicle,  he  will  observe,  is 
covered  by  a  glistening  serous  membrane,  the  tunica  vaginalis  propria, 
which  he  can  trace  upwards  on  the  cord.  This  spermatic  cord  contains 
the  vessels,  nerves,  and  excretoiy  duct  (vas  deferens)  of  the  testicle, 
Avhich  structures  descend  through  the  abdominal  wall  by  an  oblique 
passage  termed  the  inguinal  canal.  In  the  upper  part  of  this  canal, 
which  is  not  to  be  exposed  at  present,  the  tunica  vaginalis  propria  of  the 
cord  is  continuous  with  the  tunica  vaginalis  reflexa. 

The  Cremaster  Muscle  is  continuous  with  the  cremasteric  fascia 
already  described.     It  is  a  bright  red  muscle,  placed  in  the  cord  beneath 


DISSECTION    OP   THE   PERINEUM   IN   THE   MALE.  279 

its  serous  covering.  It  passes  upwards  through  the  inguinal  canal,  in 
which  its  connections  will  be  observed  at  a  later  stage. 

The  Spermatic  Vessels.  The  spermatic  artery  is  an  important  vessel 
from  the  hsemorrhage  to  which  it  may  give  rise  in  castration.  It  is 
placed  in  the  anterior  part  of  the  cord,  and  in  a  well-injected  subject  its 
remarkably  convoluted  disposition  will  be  evident  without  dissection. 
The  spermatic  veins  accompany  the  artery.  They  are  large  and  tor- 
tuous. 

The  Vas  Deferens  is  the  excretory  duct  of  the  testicle,  and  is  placed 
at  the  posterior  part  of  the  spermatic  cord,  where  it  may  be  seen  and 
felt  as  a  thick,  firm  tube. 

Directions. — The  student,  having  identified  these  different  elements  of 
the  cord,  may  practise  the  operation  of  castration  by  any  one  of  the 
common  methods,  taking  care  to  sever  the  spermatic  cord  just  above  the 
epididymis,  at  the  upper  border  of  the  testicle.  The  cord  is  to  be  left 
in  the  inguinal  canal. 

the    testicle    and    EPIDIDYMIS    (PLATES    46    AND    47). 

The  Testicle  is  the  gland  that  secretes  the  semen — the  male  fertil- 
izing fluid.  In  form  it  is  ovoid.  Its  faces,  right  and  left,  are  smooth 
and  rounded;  its  inferior  border  is  slightly  convex  and  free;  its  upper 
edge  is  nearly  straight,  and  is  related  to  the  epididymis.  Its  anterior 
extremity  shows  below  the  globus  major  of  the  epididymis  a  small  cyst- 
like body — the  peduncidated  hydatid  of  Morgagni. 

The  Epididymis  is  made  up  of  the  convolutions  of  the  excretory  tube 
of  the  testicle.  It  presents  anteriorly  an  enlargement  termed  the 
glohtis  major,  and  posteriorly  a  lesser  enlargement  termed  the  globus 
m,inor,  the  intermediate  part  being  called  the  body.  At  the  globus 
minor  the  tube  loses  its  convoluted  disposition,  and  is  continued  as  the 
vas  deferens,  which,  as  already  seen,  becomes  one  of  the  constituents  of 
the  spermatic  cord. 

Structure.  The  testicle  has  for  its  most  external  investment  the 
tunica  vaginalis  j)7'op7na.  This,  as  already  explained,  is  a  serous  mem- 
brane which  passes  on  to  the  testicle  from  the  cord,  and  is  continuous 
with  the  peritoneum  at  the  upper  opening  of  the  inguinal  canal.  It  is, 
as  it  were,  the  visceral  part  of  a  serous  membrane,  the  ttmica  vaginalis 
rejiexa — the  inner  lining  of  the  bag  in  which  the  testicle  lies  free — 
being  the  parietal  portion  of  the  same  membrane.  This  covering  is 
thin  and  transparent,  and  closely  adherent  to  the  next  covering — the 
tunica  alhuginea.  The  tunica  albuginea  is  a  complete  envelope  of  dense, 
lamellated  connective-tissue,  containing  some  fibres  of  non-sti-iped 
muscular  tissue.  Towards  the  upper  and  anterior  part  of  the  testicle,  a 
strong  process  from  the  tunica  albuginea  passes  into  the  interior  of  the 
gland.     This  is  tenned  the  corpus  Highmori,  or  mediastinum  testis;  and 


280  THE  ANATOMY  OF  THE  HORSE. 

between  it  and  the  inner  surface  of  the  tunic,  numerous  trabeculaj  pass, 
forming  a  framework  for  the  gland,  and  dividing  it  into  a  number  of 
conical  compartments,  or  lobules,  which  lodge  the  seminal  tubules.  On 
the  inner  surface  of  the  tunica  albuginea,  and  on  its  trabeculse,  the 
bloodvessels  are  distributed,  forming  the  tunica  vasculosa. 

Each  seminal  tubule  begins  either  with  a  blind  extremity,  or  by  anas- 
tomosing with  an  adjacent  tubule.  The  tubes  are  highly  convoluted 
until  they  approach  the  mediastinum,  where  they  unite  to  form  a  series 
of  straight  tubes — the  tubuli  recti,  which  enter  the  mediastinum  and 
form  in  it  a  network — the  rete  testis.  From  this  network  arise  a  number 
of  tubes  termed  the  vasa  efferentia,  which  peiforate  the  tunica  albuginea 
above  the  anterior  end  of  the  testicle.  On  leaving  the  gland,  these 
become  convoluted,  forming  little  masses  known  as  the  coni  vasculosi  ; 
and  they  then  unite  with  one  another  until  there  results  a  single  excre- 
tory tube,  whose  convolutions  make  up  the  globus  major,  body,  and 
globus  minor  of  the  epididymis.  The  seminal  tubules  are  composed  of 
a  membrana  propria  and  an  epithelial  lining.  The  epithelium  is 
arranged  in  several  layers,  and  through  the  agency  of  the  innermost 
cells — sjyermatoblast  cells — the  spermatozoa  of  the  semen  are  produced. 
The  tubuli  recti  and  rete  testis  are  lined  by  a  single  layer  of  columnar 
epithelium.  The  tubes  of  the  vasa  efferentia  and  epididymis  have  a 
wall  that  contains  non-striped  muscular  fibres,  and  they  possess  a  colum- 
nar ciliated  lining. 

THE    PREPUCE. 

The  pi-epuce,  vulgarly  called  the  "  sheath,"  is  the  involution  of  skin 
which  lodges  tlie  free  portion  of  the  penis  when  that  organ  is  non- 
erect.  In  this  condition  it  consists  of  tAvo  layers — an  external,  similar 
to  the  surrounding  integument,  with  which  it  is  continuous;  and  an 
internal,  which  is  intermediate  in  texture  between  skin  and  mucous 
membrane.  The  latter  layer  is  smooth,  destitute  of  hair,  and  provided 
with  numerous  preputial  glands,  which  secrete  a  strong-smelling  sebaceous 
material.  This  material  facilitates  the  protrusion  of  the  penis  during 
erection;  and,  ordinarily,  it  accumulates  in  considerable  amount  within 
the  prepuce.  These  two  layers  are  continuous  with  one  another  at  the 
orifice  of  the  preputial  cavity,  and  at  the  posterior  end  of  the  cavity  the 
inner  layer  is  continuous  with  the  investment  of  the  penis.  Towards 
the  orifice  of  the  perputial  cavity,  two  rudimentary  tubercle-like  teats 
are  sometimes  found.  Lay  hold  of  the  extremity  of  the  penis,  and  pull 
it  forcibly  forwards,  at  the  same  time  pulling  the  prepuce  backwards. 
This  will  obliterate  the  prepuce,  as  in  erection,  in  which  condition  the 
inner  layer  of  the  pi'epuce  becomes  a  part  of  the  covering  of  the  penis. 

Directions. — While  the  penis  is  pulled  forwards  out  of  the  prepuce, 
carry  a   mesial   incision  through  the  skin  from  the  perinaium  to  the 


DISSECTION    OF    THE    PERIN.EUM    IN    THE    MALE.  281 

entrance  of  the  prepuce,  and  reflect  the  skin  on  each  side  for  three  or 
four  inches. 

Suspensory  Ligaments  of  the  prepuce.  When  the  outer  cutaneous 
layer  of  the  prepuce  is  removed,  there  is  exposed  an  elastic  fibrous  layer 
which  descends  into  it  on  each  side  from  the  abdominal  tunic.  These 
are  the  suspensory  lij^aments  of  the  prepuce. 

Vessels  and  Nerves.  The  cutaneous  nerves  of  the  prepuce  and  scro- 
tum are  branches  of  the  inguinal  nerve  or  nerves.  One  or  more  of 
these,  derived  from  the  2nd  and  3rd  lumbar  nerves,  descend  through 
the  ingiiinal  canal.  The  arteries  are  branches  of  the  stihcutaneous 
ahdoininal  artery.  This  vessel,  which  is  a  branch  of  the  external  pudic 
artery,  passes  forwards  a  few  inches  from  the  middle  line.  The  trunk 
of  the  artery  is  to  be  left  undisturbed  at  present. 

A  rich  plexus  of  veins  exists  in  and  around  the  scrotum.  This 
plexus  is  drained  by  a  comparatively  small  vein  that  accompanies  the 
external  pvidic  artery,  and  by  a  larger  vessel  which  penetrates  the 
gracilis  to  empty  itself  into  the  femoral  vein. 


Directions, — While  the  penis  is  pulled  forwards,  reflect  the  integu- 
mental  covering  from  the  upper  face  of  its  free  portion,  and  follow  back- 
wards its  dorsal  vessels  and  nerves. 

Dorsal  Arteries  of  the  penis  (Plates  39  and  46).  On  each  side  there 
are  two  of  these,  distinguished  as  anterior  and  posterior.  1.  The  anterior 
dorsal  artery  of  the  penis  is  one  of  the  terminal  branches  of  the  external 
pudic  artery.  It  results  from  the  bifurcation  of  that  vessel  imme- 
diately after  its  emergence  from  the  inguinal  canal,  and  after  a  course 
of  a  few  inches  it  divides  into  an  anterior  branch  which  passes  forwards 
on  the  free  portion  of  the  penis,  and  a  posterior  which  passes  backwards 
on  the  fixed  portion,  meeting  and  anastomising  with  the  posterior 
dorsal  artery.  When  the  penis  is  non-erect,  the  anterior  of  these 
branches  has  a  flexuous  disposition,  which  permits  it  to  be  elongated 
without  stretching  when  the  organ  becomes  erect.  2.  The  2^osterior 
dorsal  artery  of  the  penis  is  a  branch  of  the  cavernous  artery  (from  the 
obturator).  It  runs  forwards  on  the  dorsal  aspect  of  the  fixed  portion 
of  the  penis,  and  anastomoses  with  the  posterior  division  of  the  anterior 
dorsal  artery.  These  arteries  are  mainly  expended  in  branches  to  the 
cavernous  and  spongy  portions  of  the  penis,  and  they  also  give  off  some 
twigs  to  the  prepuce. 

Dorsal  Nerves  of  the  penis.  These  nerves,  right  and  left,  accom- 
pany the  dorsal  vessels  on  the  dorsum,  or  upper  surface,  of  the  penis. 
Each  is  the  continuation  of  the  pudic  nerve,  which  reaches  the  penis  by 
turning  round  the  ischial  arch.  In  proceeding  forwards  along  the 
penis,  the  nerves  are  disposed  in  a  flexuous  manner  to  allow  them  to  be 


282  THE  ANATOMY  OF  THE  HORSE. 

adapted  without  stretching  to  the  varj'ing  length  of  the  organ.  They 
emit  numerous  branches  to  the  cavernous  and  spongy  portions  of  the 
penis,  and  terminate  in  the  glans. 

Suspensory  Ligaments  of  the  penis  (Plate  46).  These  are  two  fibrous 
bands,  right  and  left,  which  are  attached  superiorly  to  the  tendon  of 
origin  of  the  gracilis,  and  below  to  the  cavernous  body  of  the  penis. 

Directions. — The  penis  may  now  be  freed  as  far  as  its  posterior 
extremity,  and  its  surface  cleaned  of  vessels,  nerves,  and  connective- 
tissue.  On  one  side  the  erector  penis  muscle  should  be  removed,  to  lay 
bare  the  cms  and  expose  the  artery  of  the  corpus  cavernosum. 

The  Artery  of  the  Corpus  Cavernosum  (Plate  46).  This  is  a  branch 
d|f  the  obturator  artery,  detached  after  the  emergence  of  that  artery 
from  the  obturator  foramen.  It  passes  backwards  on  the  lower  face  of 
the  ischium,  and  perforates  the  crus  penis.  It  gives  off  as  a  collateral 
branch  the  posterior  dorsal  artery  of  the  penis. 

The  Penis  (Plates  46  and  47)  is  the  male  organ  of  copulation.  It 
begins  at  the  ischial  arch,  where  it  is  attached  by  its  crura  to  the  ischial 
tuberosities;  and  it  terminates  anteriorly  in  a  free  enlargement — the 
glans.  It  may  be  said  to  consist  of  a  posterior  fixed  portion,  and  an 
anterior  portion  which  is  free  and  protrusible.  The  former  portion 
extends  from  the  ischial  arch  to  the  scrotum;  the  latter,  when  the 
organ  is  non-erect,  is  lodged  in  the  prepuce,  but  during  erection  the 
prepuce  becomes  obliterated,  and  this  part  of  the  penis  then  projects 
freely  in  front  of  the  scrotum. 

The  penis  is  compounded  of  three  longitudinal  and  parallel  columns, 
viz.,  two  corpora  cavernosa  and  a  single  corpus  sp)ongiosmn.  From  the 
relationship  of  these  to  one  another,  the  penis  has  been  happily  com- 
pared to  a  double-bai-relled  gun,  the  barrels  being  represented  by  the 
corpora  cavernosa,  and  the  ramrod  by  the  corpus  spongiosum. 

The  Corpora  Cavernosa.  Each  corpus  cavernosum  begins  at  the 
tuber  ischii,  to  whose  inferior  ridge  (inferior  ischiatic  spine)  it  is  firmly 
attached  under  cover  of  the  erector  penis  muscle.  These  constitute  the 
roots,  or  crura,  of  the  penis,  and  they  converge  towards  each  other  and 
form  a  single  mass  which  makes  up  the  main  thickness  of  the  penis  as 
far  as  the  glans.  The  united  corpora  cavernosa  have  an  upper  flattened 
surface,  or  dorsum,  along  which  the  dorsal  vessels  and  nerves  pass. 
Their  sides  are  smooth  and  slightly  rounded,  and  inferiorly  they  form  a 
shallow  median  groove  for  the  corpus  spongiosum  (Fig.  44).  Anteriorly 
they  terminate  bluntly  in  the  glans. 

The  Corpus  Spongiosum  forms  a  much  more  slender  column  than  the 
corpora  cavernosa.  It  is  traversed  in  the  whole  of  its  length  by  the 
extra-pelvic  pai't  of  the  urethra.  This  urethra,  as  will  subsequently  be 
seen,  begins  at  the  neck  of  the  bladder,  and  its  first  few  inches  are 
intra-pelvic,  being  placed  over  the  ischiatic  symphysis.     Turning  round 


DISSECTION    OF    THE    PERINEUM    IN    THE    MALE.  '2q6 

the  ischial  arch,  the  intra-pelvic  urethra  becomes  directly  continuous 
with  the  extra-pelvic  portion,  and  from  the  point  of  continuity  onwards 
the  urethra  is  enveloped  in  a  sheath  of  erectile  tissue,  which  is  the 
corpus  spongiosum.  The  corpus  spongiosum  forms  at  either  of  its 
extremities  an  enlargement.  The  posterior  enlargement,  which  is 
situated  at  the  ischial  arch,  is  termed  the  bulb;  the  anterior  enlarge- 
ment is  the  glans  2)enis.  The  glans  forms  the  expanded  free  extremity  of 
the  penis,  and  it  surrounds  the  blunt  anterior  end  of  the  united  corpora 
cavernosa.  During  erection  the  enlargement  assumes  a  shape  resem- 
bling, somewhat,  the  rose  of  a  watering-can,  having  a  prominent  ridge — 
the  corona  glandis,  behind  which  there  is  a  slight  constriction — the 
cervix.  The  front  of  this  rose-like  swelling  presents  a  fossa  from  which 
the  urethra  projects  for  about  half  an  inch  as  a  free  tube — the  urethral 
tube.  Above  the  base  of  the  urethral  tube  there  is  the  opening  of  a 
double  cavity — the  urethral  sinus,  which  generally  contains  some  of  the 
partially  inspissated  secretion  of  sebaceous  glands  that  open  into  the 
cavity.  Inferiorly  the  corona  glandis  is  interrupted  on  the  middle  line 
by  the  suburethral  notch. 

The  corpus  spongiosum  as  far  as  the  glans  is  surrounded  by  the 
accelerator  urinse  muscle.  Superiorly  it  fits  into  the  groove  on  the 
lower  aspect  of  the  corpora  cavernosa,  and  along  its  under  aspect  pass 
the  retractor  muscles  of  the  penis. 

Directions. — Immediately  in  front  of  the  junction  of  its  crura,  the 
penis  should  now  be  amputated,  that  the  structure  of  its  component 
parts  may  be  examined. 

Structure  of  the  corpora  cavernosa.  The  corpora  cavernosa  possess 
a  strong  envelope  of  white  fibrous  tissue,  termed  the  tunica  albuginea. 
This  tunica  albuginea,  besides  forming  a  common  envelope  to  the  united 
bodies,  sends  inwards  an  incomplete  mesial  septum  between  the  two — 
the  septum  pectiniforme.  This  septum  when  viewed  laterally  is  seen  to 
be  perforated  by  numerous  vertical  slits,  which  give  its  processes  a 
resemblance  to  the  teeth  of  a  comb ;  hence  the  name.  Besides  the  sep- 
ti;m  pectiniforme,  numerous  small  trabeculse  pass  into  the  interior  of  the 
corpora  cavernosa,  and  by  their  anastomosis  form  a  framework  for  these 
bodies.  The  trabecule  are  composed  of  fibrous  tissue  with  some  bundles 
of  non-striped  muscular  tissue.  Between  the  trabeculse  are  innumer- 
able intercommunicating  spaces,  placed  between  the  capillaries  and  the 
small  veins.  During  erection  the  blood  is  poured  into  these  spaces, 
and  thus  is  brought  about  the  increase  in  the  size  of  the  organ.  At 
other  times  the  blood  passes  in  the  ordinary  manner  from  the  capillaries 
to  the  venous  radicles.  In  the  crura  and  peripheral  part  of  the 
cavernous  bodies  some  of  the  small  arteries  terminate  directly  in 
these  venous  spaces.  The  small  arteries  are  imbedded  in  the 
trabeculse,  and  when  these  are  contracted,  in  the  non-erect  state,  the 


284  THE  ANATOMY  OF  THE  HORSE. 

arteries  assume  a  coiled  disposition,  from  which  they  receive  the  name 
artence  helicince. 

Structure  of  the  corpus  spongiosum.  The  structure  of  the  spongy 
body  resembles,  somewhat,  that  just  described.  It  possesses  an 
envelope  of  fibrous  tissue  with  trabecula3  and  a  plexus  of  large  veins. 
In  its  peripheral  part,  and  in  the  bulb,  it  also  contains  true  cavernous 
spaces,  like  those  of  the  cavernous  bodies  but  smaller. 

Structure  of  the  spongy  (or  extra-pelvic)  part  of  the  urethra.  This 
should  be  laid  open  on  its  under  aspect  with  scissors.  The  lumen  of 
the  tube  is  not  uniform.  At  the  ischial  arch  (this  will  not  be  seen  at 
present)  it  presents  a  dilatation ;  and  its  calibre  is  again  increased  as  it 
enters  the  glans,  forming  what  is  termed  in  man  the  fossa  navicularis. 
The  interior  of  the  tube  is  lined  by  mucous  membrane  having  simple 
columnar  epithelium,  except  at  its  orifice,  where  it  is  stratified  and 
squamous.  The  ducts  of  numerous  small  racemose  glands  open  on  the 
surface  of  the  membrane.  External  to  the  mucous  membrane  the  wall 
of  the  urethra  is  made  up  of  non-striped  muscular  tissue,  an^anged  as  an 
inner  circular  and  an  outer  longitudinal  layer. 


CHAPTER    X. 

DISSECTION  OF  THE  ABDOMEN. 

Before  this  part  can  be  begun  in  the  male  subject,  the  dissection  of 
the  perinseum  (Chapter  IX.)  must  be  completed. 

THE    ABDOMINAL    WALL, 

Position. — The  subject  should  be  placed  on  the  middle  line  of  its 
back,  or  slightly  inclined  to  one  side,  its  limbs  being  drawn  upwards  and 
outwards  by  ropes  and  pulleys. 

The  Mammary  Glands,  or  the  Udder.  It  is  convenient  to  describe 
here  these  glands,  since  their  dissection  must  precede  that  of  the 
abdominal  wall.  They  are  organs  peculiar  to  the  female,  occupying  the 
position  of  the  scrotum  in  the  male.  As  regards  their  function,  they 
may  be  viewed  as  an  accessory  part  of  the  reproductive  system,  secreting 
the  milk  upon  which  the  young  animal  subsists  for  some  time  after 
birth.  It  is  only  during  the  period  of  lactation  that  they  become  fully 
developed,  and  therefore  a  subject  suited  for  the  satisfactory  display 
of  their  structure  seldom  presents  itself  in  the  dissecting-room. 

The  glands  are  two  in  nvimber,  and  are  placed  side  by  side  on  the 
middle  line  of  the  abdominal  wall,  in  front  of  the  pubes.  They  form 
here  a  single  mass,  with  a  wide  and  shallow  mesial  furrow  between 
them.  The  term  "  udder  "  is  used  to  include  both  glands.  From  the 
most  prominent  part  of  each,  the  mamilla,  teat,  or  nijjple,  projects. 
This  has  the  form  of  a  short,  flattened  cone.  Its  free  extremity  is 
perforated  by  two  or  three  orifices  belonging  to  the  lai'ge  milk  ducts  by 
Avhich  the  milk  is  extracted  from  the  gland.  The  integumentary 
covering  of  both  glands  and  teats  is  thinner  than  the  surrounding  skin, 
and  it  is  generally  black-pigmented.  Moreover,  the  ordinary  body  haii-s 
are  absent  over  it,  their  place  being  taken  by  a  fine  down,  except  over 
the  summit  of  the  teat,  where  there  are  no  hairs.  It  is  richly  provided 
with  sebaceous  and  sudoriparous  glands,  whose  secretion  renders  it 
moist. 

When  the  cutaneous  covering  of  the  gland  is  reflected,  there  is 
exposed  a  second  envelope,  composed  of  yellow  elastic  tissue.  This 
covering  detaches  a  number  of  processes  into  the  interior  of  the  gland 


286  THE  ANATOMY  OF  THE  HORSE. 

between  its  main  lobes,  and  on  the  mesial  plane  the  elastic  envelopes 
of  the  two  glands  are  applied  together,  and  form  a  kind  of  intermediate 
septum.  A  few  strong  slips  of  the  same  texture  descend  into  the  gland 
from  the  abdominal  tunic,  and  play  the  part  of  suspensory  ligaments. 

The  secretory  sti'ucture  of  the  gland  is  arranged  on  the  racemose 
type.  If  a  bristle  be  passed  into  one  of  the  orifices  seen  at  the  extremity 
of  the  teat,  it  will  pass  upwards  by  the  large  milk  duct,  and  enter  a 
dilatation  at  the  base  of  the  teat,  termed  the  galactopherous  or  lactiferous 
sinus.  The  secretion  of  milk  during  the  period  of  lactation  is  constant, 
and  the  liquid  accumulates  in  these  reservoirs,  to  be  drawn  off  by  the 
young  animal.  The  milk  ducts  and  the  sinuses  are  lined  by  a  mucous 
membrane;  and  in  the  substance  of  the  teat,  between  this  mucous  lining 
and  the  external  skin,  there  are  some  fibres  of  non-striped  muscular 
tissue,  arranged  both  longitudinally  and  circularly.  The  circular  fibres 
prevent  the  escape  of  the  milk  from  the  sinus. 

The  milk  enters  each  sinus  from  a  number  of  tubes  \yhich,  when 
traced  into  the  substance  of  the  gland,  divide  and  subdivide ;  and  the 
smallest  ducts  resulting  from  this  subdivision  lead  up  to  the  ultimate 
acini  of  the  gland  structure.  These  acini  are  lined  by  a  secretory 
epithelium  by  Avhose  agency  the  milk  is  formed. 

The  arteries  and  veins  of  the  glands  are  branches  of  the  external 
pudic  vessels.  They  undergo  a  great  increase  in  size  during  lactation. 
The  nerves  of  the  gland  are  branches  of  the  inguinal  nerves. 

Directions. — Reflect  the  skin  as  shown  in  Plate  38.  If  the  dissector 
of  the  fore  limb  be  engaged  with  the  pectoral  region,  the  skin  from  the 
posterior  part  of  that  region  will  be  turned  back  in  a  piece  with  that 
over  the  front  of  the  abdomen.  If  not,  the  dissector  of  the  abdomen 
must  limit  the  skin  which  he  is  about  to  reflect,  by  an  incision  earned 
outwards  from  the  ensiform  cartilage  to  the  point  of  the  elbow.  He 
will  be  guided  in  the  same  way  towards  the  hind  limb.  Care  must  be 
taken  not  to  reflect  the  panniculus  with  the  skin. 

A  slight  degree  of  tjonpanitic  distension  of  the  intestines  is  favourable 
for  the  dissection  of  the  abdominal  wall.  When  excessive,  however,  as 
it  often  becomes,  it  interferes  with  the  dissection,  and  is  almost  certain 
to  rupture  the  diaphragm,  or  the  abdominal  wall  before  its  dissection 
can  be  completed.  This  should  be  pi-evented  by  tapping  the  large 
intestine  with  a  canula  and  trochar,  making  the  puncture  at  the  most 
prominent  part. 

Cutaneous  Nerves.  In  reflecting  the  skin,  a  multitude  of  small 
nerves  will  be  seen  on  its  inner  sui-fiice.  They  are  derived  from  the 
intercostal  nerves. 

The  Subcutaneous  Abdominal  Artery  (Plate  38).  Look  for  this 
vessel  near  the  middle  line,  in  the  region  of  the  prepuce  or  mammarj' 
gland.     It  is  one  of  the  terminal  divisions  of  the  external  pudic  artery, 


DISSECTION    OF    THE    ABDOMEN.  287 

and  is  distributed  to  the  sci'otum  and  prepuce  (skin  of  mammary  gland 
in  mare),  superficial  inguinal  glands,  and  skin,  terminating  a  little  in 
front  of  the  umbilicus. 

The  Subcutaneous  Abdominal  Vein  runs  in  company  with  the  artery. 

The  Superficial  Inguinal  Lymphatic  Glands  (Plate  38).  These 
form  a  small  group  close  to  the  subcutaneous  abdominal  artery,  at  the 
side  of  the  prepuce. 

The  Subcutaneous  Thoracic  (Spur)  Vein  (Plate  38)  will  be  found 
on  the  surface  of  the  panniculus.  The  primary  rootlets  of  the  vein 
collect  blood  from  the  skin  in  front  of  the  mamma  or  prepuce,  and  pass 
on  to  the  surface  of  the  panniculus,  where  they  unite  to  form  the  trunk 
of  the  vein.  This  is  at  first  lodged  in  a  groove  on  the  superficial  aspect 
of  the  paimiculus.  It  then  perforates  the  muscle  ;  and  gaining  its  deep 
face,  it  passes  forwards  towards  the  axilla  (Plate  1),  where  it  joins  the 
brachial  vein.  The  course  of  this  vein  is  usually  distinctly  visible  in  the 
living  animal.  From  its  position  it  is  liable  to  be  injured  in  deep 
spurring,  and  hence  one  of  its  names. 

The  Panniculus  Carnosus  (Plate  38).  This  is  a  thin  extended  sheet 
of  muscular  tissue,  which  is  adherent  to  the  deep  surface  of  the  skin 
over  a  large  part  of  the  abdomen  and  thorax,  being  continued  also  from 
the  latter  region  over  the  outer  aspect  of  the  shoulder.  The  most 
posterior  angle  of  the  muscle  is  included  in  the  fold  of  skin  at  the  groin, 
but  it  does  not  reach  the  hind  limb.  From  this  angle  the  superior  edge 
of  the  muscle  (which  will  not  at  present  be  seen)  slopes  upwards  with 
two  or  three  wide  sinuosities  to  near  the  spine  in  the  doi'sal  region, 
while  from  the  same  point  the  posterior  edge  of  the  muscle  slopes  down- 
wards and  forwards  to  a  second  angle  which  is  rounded  and  placed  from 
three  to  six  inches  external  to  the  umbilicus.  The  inferior  edge  extends 
from  this  latter  angle  forwards  towards  the  elbow.  Anteriorly  the 
muscle  is  continued  over  the  scapular  region,  and  sends  also  an  apon- 
eurotic tendon  between  the  fore  limb  and  the  chest-wall  to  be  attached 
to  the  internal  tuberosity  of  the  humerus.  The  edges  of  the  muscle  are 
prolonged  by  a  thin  fascia  which  is  attached  superiorly  to  the  vertebral 
spines,  and  below  and  behind  is  adherent  to  the  abdominal  tunic.  The 
outer  surface  of  the  muscle  is  with  difticulty  separated  from  the  skin, 
which  indeed  receives  the  insertion  of  its  fibres.  The  muscular  tissue 
of  the  panniculus  is,  as  compared  with  striped  muscles  in  general,  of  a 
pale  colour. 

Action. — It  twitches  the  skin,  and  plays  the  part  of  a  hand  to  the 
animal  in  removing  offending  insects. 

Directions. — Begin  at  the  lower  edge  of  the  panniculus  and  raise  it 
upwards  from  the  subjacent  structures.  This  is  easy  over  the  abdo- 
minal tunic,  but  anteriorly  it  is  closely  adherent  to  the  edge  of  the 
deep  pectoral  muscle.     The  panniculus  is  not  to  be  removed,  but  raised 


288  THE  ANATOMY  OF  THE  HORSE. 

as  fiir  as  is  necessary  to  bring  the  origin  of  the  external  ol)liq\ie  muscle 
of  the  abdomen  into  view.  Notice  on  the  inner  surfiice  of  the  muscle 
ramifying  nerves,  and  anteriorly  the  spur  vein  accompanied  by  a  small 
branch  of  the  external  thoracic  artery. 

Perforating  Nerves.  The  nerves  seen  descending  on  the  inner 
surface  of  the  panniculus  are  perforating  branches  derived  from  the 
intercostal  trunks,  and  from  the  last  dorsal  and  first  lumbar  nerves. 
These  perforating  nerves  appear  along  a  curved  line  a  few  inches  below 
the  origin  of  the  external  oblique.  They  supply  the  panniculus,  and  give 
cutaneous  twigs  through  it  to  the  overlying  skin. 

A  perforating  branch  from  the  2nd  lumbar  nerve  appears  close  to  the 
bony  prominence  of  the  haunch,  and  descends  to  the  skin  on  the  h'oni, 
of  the  thigh,  A  perforating  branch  from  the  3rd  lumbar  nerve  appears 
below  the  same  bony  prominence,  and  two  inches  below  the  point  of  exit 
of  the  preceding  nerve.  It  is  accompanied  by  a  branch  of  the  circum- 
flex iliac  artery,  with  which  it  descends  to  the  thigh,  internal  to  the  last 
described  branch. 

The  Subcutaneous  Thoracic  Nerve  (Plate  1).  This  will  be  found 
ninning  horizontally  backwards  on  the  inner  surface  of  the  panniculus, 
behind  the  shoulder,  and  in  company  with  the  vessels  of  the  same  name. 
It  comes  from  the  brachial  plexus. 

Perforating  Vessels.  Small  un-named  branches,  mostly  branches 
of  the  intercostal  vessels,  appear  at  the  same  points  as  the  nerves. 

The  Abdominal  Tunic  (Plate  38).  This  is  a  great  expansion  of 
yellow  elastic  tissue  which  is  spread  over  the  inferior  and  lateral  walls 
of  the  abdomen.  It  is  nearly  co-extensive  with  the  external  oblique 
muscle,  to  which  it  is  adherent.  It  is  thickest  in  its  posterior  part, 
near  the  linea  alba ;  and  becomes  gradually  tliinner  as  it  is  tx'aced  out- 
wards over  the  muscular  part  of  the  external  oblique,  and  forwards 
beneath  the  posterior  deep  pectoral.  Posteriorly  it  furnishes  the  sus- 
pensory ligaments  of  the  prepuce,  or  analogous  slips  to  the  mammaiy 
gland.  The  tunic  acts  as  an  admirable  clastic  abdominal  bandage, 
assisting  the  muscles  to  support  the  heavy  abdominal  viscera,  and 
adapting  the  wall  of  the  abdomen  to  the  varying  volume  of  its  contents. 

Directions. — The  abdominal  tunic  must  be  entirely  removed.  This  is 
an  operation  requiring  time  and  care,  for  the  tunic  is  intimately  adherent 
to  the  tendon  of  the  external  oblique  muscle,  especially  in  its  anterior 
half.  Transverse  incisions  should  be  made  through  it,  taking  care  not 
to  cut  the  fibres  of  the  subjacent  tendon,  which  will  be  recognised  by  its 
different  colour  and  texture.  Then  seize  the  cut  edges  of  the  tunic  with 
the  forceps,  and  tear  it  off  in  strips  forwards  and  backwards.  Proceed 
in  this  way  until  the  whole  of  it  has  been  torn  away. 

Muscles  of  the  Abdominal  Wall.  On  each  side  there  are  four  of  these, 
viz.,  the  obliquus  abdominis  extenius,  the  obliquus  abdominis  intemus, 


DISSECTION    OF    THE    ABDOMEN.  289 

the  rectus  abdominis,  and  the  transversalis  abdominis.  They  are  stated 
in  the  order  of  their  occurrence,  the  first  being  the  most  external. 
These  muscles  have  not  only  to  discharge  the  ordinary  function  of 
a  muscle,  but  they  have  also  to  close  in  the  abdominal  cavity ;  and  fur 
this  latter  purpose,  they  are,  with  the  exception  of  the  rectus  abdominis, 
peculiarly  modified  in  form.  Thus,  the  two  oblique  muscles  and  the 
transverse  muscle  have  their  tendons  of  insertion  extended  in  the  form 
of  great  fibrous  or  aponeurotic  sheets,  and  the  fibres  in  each  of  these 
tendons  have  a  direction  different  from  that  of  the  others. 

The  LiNEA  Alba  is  the  white  mesial  raphe,  or  band,  which  extends  from 
the  ensiform  cartilage  to  the  pubes.  It  is  fibrous  in  structure,  and  is 
formed  by  the  meeting  of  the  aponeiirotic  tendons  of  the  right  and  left 
muscles.  A  little  behind  its  mid  point  is  a  puckered  cicatrix — the 
timhilicus. 

The  External  Abdominal  Ring  (Plate  39).  This  is  the  lower  orifice 
of  the  inguinal  canal.  It  has  the  form  of  a  slit  in  the  tendon  of  the 
external  oblique.  The  direction  of  the  slit  is  oblique  forwards  and  out- 
wards. The  lips,  or  2)iflars,  of  the  slit  are  simply  fibres  of  the  external 
oblique  tendon.  The  inner  angle  or  commissm'e  is  placed  at  the  edge  of 
the  prepubic  tendon.  This  prepubic  tendon  is  a  strong  fibrous  band  by 
which  the  abdominal  muscles  get  a  common  insertion  into  the  anterior 
edge  of  the  pubic  bones,  and  from  whose  surface  the  pubio-femoral  liga- 
ment of  the  hip-joint  arises.  The  external  abdominal  ring  gives  passage 
in  the  male  to  the  spermatic  cord,  the  external  pudic  vessels,  and  the 
inguinal  nerves.  In  the  female  it  transmits  merely  the  corresponding 
vessels  and  nerves. 

The  Obliquus  Abdominis  Externus  (Plate  39).  This  consists  of 
a  mviscular  band  at  its  antei'o-superior  edge,  and  an  aponeurotic  tendon 
over  the  inferior  and  lateral  parts  of  the  abdomen.  It  arises  by  its 
muscular  portion  from  the  outer  surfixce  of  the  last  fourteen  ribs,  and 
behind  the  last  rib  from  the  tendon  of  the  latissimus  dorsi.  Its  anterior 
slips  of  origin  interdigitate  with  the  serratus  magnus.  The  muscular 
fibres  are  directed  ol^liquely  downwards  and  backwards,  and  are  suc- 
ceeded by  the  aponeurotic  tendon.  The  fibres  of  the  tendon  continue  in 
the  same  direction,  and  become  inserted  into  the  linea  alba,  the  prepubic 
tendon,  and  the  external  angle  of  the  ilium ;  while  between  the  two  last- 
mentioned  points  they  are  continued  to  form  Fouparfs  ligament.  Along 
the  line  between  these  two  points  the  fiiscia  of  the  inside  of  the  thigh  is 
inserted  to  the  surface  of  the  tendon,  and  it  must  be  cut  in  order  to 
expose  the  ligament.  It  will  then  be  observed  that  from  the  prepubic 
tendon  to  the  bony  prominence  of  the  haunch,  the  fibres  of  the  external 
oblique  tendon,  instead  of  becoming  inserted  into  bone,  curve  upwards 
and  forwards  and  are  lost  to  view.  It  is  these  reflected  fibres  that 
constitute  the  ligament  of  Powpart  (Plate  40),  which  may  be  described 

u 


290  THE  ANATOMY  OF  THE  HORSE. 

iis  having  two  extremities,  two  sui-faces,  and  two  edges.  Its  extremities 
are  attached  to  the  pubis  and  angle  of  the  haunch  respectively.  Its 
anterior  surface  is  concave,  and  directed  towards  the  abdomen.  This 
surfiice  gives  origin  outwardly  to  fibres  of  the  internal  oblique  muscle, 
and  inwardly  it  foi-ms  the  posterior  wall  of  the  inguinal  canal.  The 
posterior  surface  is  convex,  and  forms  an  arch  over  the  femoral  vessels, 
the  crural  nerve,  and  the  sartorius,  iliacus,  and  psoas  magnus  muscles 
(Plate  13).  Neither  of  the  edges  of  the  ligament  has  a  distinct  [exist- 
ence. The  posterior  or  inferior  edge  is  the  line  of  continuity  between 
the  ligament  and  the  tendon  of  the  external  oblique.  At  its  anterior  or 
superior  edge  the  ligament  becomes  thin  in  texture,  and  disappears  on 
the  fascia  covering  the  sublumbar  muscles.  All  of  these  points  cannot 
be  made  out  at  present,  but  they  will  become  evident  as  the  dissection 
proceeds. 

Action  of  the  external  oblique  muscle. — When  the  right  and  left 
muscles  act  in  concert,  they  bend  the  tnink,  and  arch  the  back.  If  the 
spine  is  fixed,  they  p\ill  the  ribs  backwards  and  assist  in  expiration.  If 
both  the  spine  and  ribs  are  fixed,  they  compress  the  abdominal  viscera, 
and  assist  in  urination,  defeecation,  and  parturition.  If  only  one  muscle 
acts,  it  bends  the  trunk  or  pelvis  to  the  same  side. 

The  Inguinal  Canal  is  the  oblique  passage  in  the  abdominal  w^all 
through  which  the  testicle  descends  in  the  young  animal,  and  in  which 
the  spermatic  cord  is  lodged  in  the  adult.  The  extetiuxl  abdominal  ring, 
which  has  already  been  examined,  is  the  lower  opening  of  the  canal. 
Its  upper  orifice,  which  will  be  seen  at  a  later  stage,  is  termed  th^' 
internal  abdominal  rinff.  The  dii'ection  of  the  canal  is  oblique  down- 
wards and  inwards,  and  it  is  slightly  curved  wath  the  concavity  forwards. 
Introduce  the  finger  into  the  canal  and  press  on  the  posterior  wall. 
This,  it  will  be  seen,  is  formed  by  the  reflected  portion  of  the  external 
oblique  tendon — in  other  words,  by  Poupart's  ligament.  Kotate  the 
hand,  and  press  the  finger  on  the  anterior  wall,  at  the  same  time  separ- 
ating the  edges  of  the  external  abdominal  ring.  The  anterior  wall  will 
be  seen  and  felt  to  be  formed  by  muscular  substance,  viz.,  by  the 
muscular  part  of  the  internal  oblique. 

The  canal  gives  passage  in  the  male  to  the  spermatic  cord,  the  external 
pudic  vessels,  and  the  inguinal  nerves.  In  the  female  it  is  much 
smaller,  and  transmits  the  corresponding  vessels  and  nerves. 

The  Spermatic  Cord.     See  page  278. 

The  External  Pudic  Artery  (Plate  39)  is  one  of  the  terminal 
divisions  of  the  prepubic.  In  the  inguinal  canal  it  descends  posterior 
and  internal  to  the  spermatic  cord.  After  its  emergence  it  divides  into 
the  subcutaneous  abdominal  arteiy,  and  the  anterior  dorsal  artery  of 
the  penis.  In  the  mare  the  latter  branch  is  represented  by  the  mammary 
artery. 


DISSECTION    OF    THE    ABDOMEN.  291 

The  External  Pudic  Vein  is  propoi-tionally  smaller  than  the  ai-tery, 
which  it  accompanies. 

The  Inguinal  Nerves  are  derived  from  the  2nd  and  3rd  lumbar  nerves, 
and  are  distributed  to  the  prepuce,  the  scrotum,  and  the  adjacent  skin. 

Directions. — Incise  the  external  oblique  tendon,  from  the  external 
angle  of  the  ilium  to  the  edge  of  the  prepubic  tendon.  Reflect  Poupart's 
ligament  towards  the  thigh,  and  hook  it  up  after  the  manner  of  Plate  40. 
Then  strip  away  the  tendon  of  the  external  oblique  from  the  subjacent 
internal  oblique.  This  Avill  be  found  easy  in  the  region  of  the  flank, 
Avhere  the  tendon  is  related  to  the  muscular  part  of  the  internal  oblique  ; 
but  over  the  inferior  part  of  the  abdomen,  and  especially  in  front,  where 
the  tendons  of  the  two  muscles  are  applied  to  each  Other,  the  opera- 
tion is  difiicult,  and  in  some  parts  impossible.  In  this  proceeding  the 
dissector  has  to  guard  against  removing  the  thin  tendon  of  the  inner 
muscle  along  with  the  outer,  and  this  he  will  best  do  by  observing  that 
the  fibres  of  the  inner  tendon  cross  these  of  the  outer  at  right  angles, 
being  directed  downwards  and  forwards.  Observe  that  anteriorly  the  two 
tendons  are  not  simply  in  apposition,  but  actually  interwoven — a  disposi- 
tion of  tendons  which  is  unique,  and  one  which  greatly  increases  the 
strength  of  the  abdominal  floor.  The  muscular  portion  of  the  external 
oblique  should  be  raised  as  far  as  the  lower  extremities  of  the  ribs.  A 
better  view  of  the  inguinal  canal  and  its  contents  will  now  be  obtained. 

The  Obliquus  Abdominis  Internus  (Plate  40)  consists  of  a  fan-shaped 
fleshy  portion  situated  in  the  flank,  and  an  aponeurotic  tendon  spread 
over  the  abdominal  floor.  It  arises  from  the  external  angle  of  the  ilium, 
and  from  the  adjacent  part  of  Poupart's  ligament.  It  is  inserted  into 
the  prepubic  tendon  and  the  linea  alba  by  the  inferior  edge  of  its  tendon, 
and  by  tendinous  slips  into  the  four  or  five  last  costal  cartilages.  In 
front  of  the  loAver  end  of  the  fourth  last  intercostal  space,  the  aponeurotic 
tendon  has  a  free  edge  which  ordinarily  lies  under  concealment  of  the 
line  of  overlapping  costal  cartilages.  When  the  abdomen  is  tym- 
panitic, however,  this  edge  is  thrust  outwards,  and  the  transversalis 
muscle  is  exposed  as  in  Plate  40.  The  jDOsterior  edge  of  the  fixn-like 
muscular  portion  lies  in  contact  with  Poupart's  ligament ;  and  the 
inguinal  canal,  as  already  seen,  passes  between  the  two  structures. 
The  highest  fibres  of  its  muscular  part  are  parallel  to  the  edge  of  a 
small  muscle — the  retractor  costoe — inserted  into  the  last  rib,  under 
cover  of  the  most  posterior  slip  of  the  serratus  posticus.  This  is 
described  with  the  muscles  of  the  back  (page  96). 

Action. — Similar  to  that  of  the  external  oblique. 

Directions. — The  internal  oblique  covers  the  transversalis  and  rectus 
abdominis  muscles.  The  outer  edge  of  the  last  may  be  seen  through 
the  thin  tendon  of  the  internal  oblique,  and  through  the  same 
tendon  the   posterior   abdominal   artery  may  be   seen   if  well   injected 


292  THE  ANATOMY  OP  THE  HORSE. 

(Plate  40).  The  circumflex  iliac  artery  is  on  the  deep  surface  of  its 
muscular  portion.  In  order  to  see  these  connections  of  the  muscle  to 
the  most  advantage,  incise  the  muscle  along  the  line  of  junction  of  the 
muscular  fan  and  the  tendon.  Raise  the  muscular  portion  carefully, 
and  hook  it  back.  Strip  away  entirely  the  aponeurotic  tendon,  using 
the  scalpel  where  the  tendon  is  firmly  adherent  to  the  rectus  abdominis. 

The  Circumflex  Iliac  Artery.  This  is  a  branch  of  the  external 
iliac  artery,  and  will  be  better  seen  in  the  dissection  of  the  sublumbar 
region  (Plate  44).  It  has  an  antenor  division  whose  branches  are 
distributed  to  the  internal  oblique  and  transverse  muscles  in  the  flank, 
and  a  2^osterior  division  which,  after  giving  some  twigs  to  the  oblique 
muscles,  perforates  them  below  the  angle  of  the  haunch,  and  descends  to 
the  thigh. 

The  Posterior  Abdominal  Artery  (Plate  40).  This  is  a  branch  of 
the  prcpubic  artery,  beginning  at  the  inner  side  of  the  internal  abdominal 
ring.  It  places  itself  on  the  abdominal  aspect  of  the  internal  oblique 
muscle,  crosses  behind  and  internal  to  the  ring,  and  runs  forwards  to 
enter  the  rectus  abdominis,  in  which,  about  midway  between  the  sternum 
and  the  pubis,  it  anastomoses  with  the  anterior  abdominal  artery. 

These  arteries  are  accompanied  by  veins  of  the  same  names. 

The  Rectus  Abdominis  (Plate  40).  This  muscle  extends  in  the 
form  of  a  broad  band  from  the  sternum  to  the  pubis,  at  the  side  of 
the  linea  alba.  To  a  large  extent  it  separates  the  internal  oblique  and 
transverse  muscles,  but  beyond  its  outer  border  these  muscles  are 
in  contact  in  the  flank  and  below  the  extremities  of  the  ribs.  The 
umscle  is  widest  about  its  centre,  and  it  is  crossed  from  side  to  side  by 
a  number  (about  a  dozen)  of  white  lines — linece  transversoi,  which  are 
caused  by  as  many  tendinous  intersections  of  its  muscular  substance. 
It  arises  from  the  lower  face  of  the  sternum,  and  from  the  five  costal 
cartilages  behind  the  4th.  It  is  inserted  into  the  anterior  border  of  the 
pubis  by  the  prepubic  tendon. 

Action. — Similar  to  that  of  the  oblique  muscles. 

Nerves  (Plate  40).  At  the  lower  ends  of  the  last  ten  intercostal 
spaces,  the  intercostal  nerves  are  prolonged  beyond  the  rim  of  overlapping 
cartilages  to  pass  between  the  straight  and  transverse  muscles,  giving 
fibres  to  both  and  also  some  perforating  twigs  to  I'each  the  skin.  The 
last  dorsal  nerve  (behind  the  last  rib)  has  a  similar  distribution.  The 
inferior  primary  branches  of  the  1st  and  2nd  lumbar  nerves  are  similarly 
prolonged  after  furnishing  twigs  to  the  oblique  muscles  in  the  flank. 

Directions. — Cut  the  rectus  abdominis  transversely  about  the  um- 
bilicus, and  reflect  it  forwards  and  backwards  from  the  subjacent  trans- 
versalis.     Look  for  the  anterior  abdominal  artery  on  its  deep  face. 

The  Anterior  Abdominal  Artery  is  one  of  the  terminal  branches 
of  the  internal  thoracic  artery.       It  appears  at  the  side   of  the   ensi- 


DISSECTION    OF    THE   ABDOMEN.  293 

form  cartilage,  where  it  turns  round  the  9th  costal  cartilage  behind  its 
tip.  It  runs  backwards  along  the  middle  of  the  superior  face  of  the 
rectus,  giving  off  lateral  branches,  and  terminating  aboiit  midway 
between  the  sternum  and  pubis  in  branches  which  anastomose  with 
those  of  the  posterior  abdominal  artery.  It  is  accompanied  by  a 
satellite  vein. 

The  Transversalis  Abdominis  (Plate  40).  This  muscle  consists 
of  a  fleshy  band  at  its  origin,  and  of  an  aponeurotic  tendon  over 
the  abdominal  floor.  In  both  of  these  the  direction  of  the  fibres  is 
transversely  downwards  and  inwards  towards  the  linea  alba.  It  arisen 
by  its  fleshy  portion  from  the  lower  extremities  or  cartilages  of  the 
asternal  ribs  (last  ten),  meeting  here  the  origin  of  the  diaphragm ;  and 
from  the  transverse  processes  of  the  lumbar  vertebrse.  It  is  inserted 
by  the  inner  edge  of  the  aponeurotic  tendon  into  the  ensiform  cartilage 
and  the  linea  alba.  The  posterior  edge  of  the  tendon  is  thin  and  ill- 
defined.  The  inner  surfixce  of  the  entire  muscle  is  related  to  the 
parietal  peritoneum,  there  being  interposed,  however,  a  very  thin  layer 
of  connective-tissue  representing  the  fascia  transversalis  of  man.  Slender 
branches  from  the  intercostal  or  asternal  vessels  run  on  the  peritoneal 
surface  of  the  muscle. 

Action. — Similar  to  that  of  the  oblique  muscles. 

Directions. — The  abdominal  cavity  will  be  exposed  by  the  removal  of 
the  transverse  muscle  and  its  peritoneal  lining.  If  only  one  side  of  the 
abdominal  wall  has  been  dissected,  the  other  side  may  now  be  used  for 
the  better  display  of  things  not  satisfactorily  made  out  in  the  first;  and 
particularly,  a  portion  of  the  abdominal  wall  in  front  of  Poupart's  liga- 
ment should  be  turned  back  in  its  entire  thickness,  so  as  to  expose  its 
peritoneal  aspect  and  the  internal  abdominal  ring. 

The  Internal  Abdominal  Ring  (Plate  44)  is  the  abdominal  opening 
of  the  inguinal  canal.  As  seen  from  the  abdominal  side,  its  posterior 
or  outer  edge  is  prominent,  and  corresponds  to  the  edge  of  the  mviscular 
part  of  the  internal  oblique ;  while  the  opposite  boundary  of  the  ring  is 
flattened  over  the  sublumbar  muscles  covered  by  the  continuation  of 
Poupart's  ligament. 

The  student  can  now  see  the  direct  continuity  between  the  peritoneum 
and  the  tunica  vaginalis,  the  latter  membrane  passing  directly  into  the 
inguinal  canal,  and  forming  a  well-defined  edge  on  the  posterior  and 
outer  side  of  the  entrance.  It  is  by  this  opening  that  a  portion  of 
intestine  or  mesentery  sometimes  passes  into  the  inguinal  canal,  or 
onwards  into  the  scrotum,  constituting  an  inguinal  or  scrotal  hernia. 

The  Prepubic  Artery  (Plate  44).  This  vessel  arises  from  the  femoral 
artery  at  the  brim  of  the  pubis,  forming  a  short  common  trunk  with  the 
deep  femoral  branch.  It  crosses  to  the  edge  of  the  internal  oblique,  and 
divides  into  the  external  pudic  and  posterior  abdominal  arteries.      The 


294:  THE  ANATOMY  OF  THE  HORSE. 

former  enters  the  inguinal  canal  at  a  point  internal  to  the  internal 
abdominal  ring.  The  latter  passes  behind  the  ring,  and  crosses  it  on 
the  inner  side.  Both  branches  have  already  been  followed,  but  the 
relation  of  the  posterior  abdominal  artery  to  the  ring  should  now  be 
specially  noted,  as,  in  consequence  of  its  position,  an  incision  for  the 
relief  of  a  strangulated  hernia  must  be  made  outwards  to  avoid  wound- 
ing the  vessel. 

The  Spermatic  Cord.  The  various  structures  which  compose  the 
spermatic  cord  meet  at  the  internal  abdominal  ring.  The  va^  deferens 
is  seen  turning  inwards  to  enter  the  pelvis,  and  projecting  the  peri- 
toneum to  form  a  small  band,  or  frsenum,  for  itself.  The  vessels  and 
nerves  of  the  cord  are  to  be  left  undisturbed,  so  that  they  may  be  followed 
to  their  source  at  a  later  stage. 

The  Cremaster  Muscle  (Plate  44).  The  fibres  of  this  muscle  are 
now  seen  at  their  origin  from  the  iliac  fascia,  Avhere  they  ai*e  close  to  the 
muscular  fibres  of  the  internal  oblique.  They  pass  into  the  inguinal 
canal,  where,  separating  but  remaining  connected  b}'  intermediate 
areolar  tissue,  they  constitute  the  cremasteric  covering  of  the  cord  and 
testicle.  When  the  muscle  contracts,  it  twitches  the  testicle  upwards 
by  shortening  the  spermatic  cord. 

the  cavity  of  the  abdomen. 

Boundaries  of  the  Cavity. — The  abdomen  is  the  largest  of  the  visceral 
cavities  of  the  body.  It  is  placed  behind  the  thorax,  from  which  it  is 
separated  by  the  diaphragm;  posteriorly  it  is  directly  continuous  with 
the  cavity  of  the  pelvis;  laterally  and  inferiorly  it  is  enclosed  by  muscu- 
lar, tendinous,  and  elastic  textures  making  up  what  is  generally  termed 
the  abdominal  wall;  and  superiorly  it  is  boimded  by  the  lumbar  portion 
of  the  spine  clothed  by  the  sublumbar  muscles. 

Contents  of  the  Cavity. — The  cavity  is  occupied  mainly  by  the  gastro- 
intestinal part  of  the  alimentary  tube,  and  its  associated  glands — the 
liver  and  the  pancreas.  Besides  these,  it  lodges  the  spleen  and  the 
kidneys.  In  the  female  it  contains  the  ovaries  and  the  uterus  (in  part), 
and  in  the  male  the  vas  deferens  passes  through  it. 

Divisions  of  the  Cavity. — As  a  matter  of  convenience  in  describing  the 
position  of  its  contained  organs,  the  cavity  is  arbitrarily  divided  into  the 
following  nine  areas  : — 

left  hjiiochonilriac  epigastric  right  hyiiochondriac 

left  lumbar  umbilical  right  lumbar 

left  iliac  hyiiogastric  right  iliac 

This  siibdivision  is  quite  arbitrary,  the  boundaries  between  these  areas 
being  certain  imaginary  planes.  Tims,  the  thi'ee  anterior  regions  are 
separated  from  the  three  middle  regions  by  a  transverse  vertical  plane 
passing  through  the  lower  end  of  the  15th  rib,  and  the  three  middle 


DISSECTION   OF   THE   ABDOMEN.  295 

regions  are  separated  from  the  three  posterior  regions  by  another  trans- 
verse vertical  phxne  passing  through  the  external  angle  of  the  ilium 
(angle  of  the  haunch).  Again,  each  of  these  three  regions — anterior, 
middle,  and  posterior — is  further  subdivided  into  a  central  and  two 
lateral  regions,  this  subdivision  being  effected  by  two  vertical  and 
parallel  longitudinal  planes,  each  passing  throi;gh  the  centre  of  Poupart's 
ligament. 

Directions.^-The  intestines  of  the  horse,  owing  to  their  unwieldy  size, 
and  generally  also  to  the  weight  of  their  contents,  are  extremely  incon- 
venient to  dissect.  From  the  following  description  and  the  accompany- 
ing plates,  the  student  should  first  learn  how  the  intestinal  tube  is 
divided.  He  should  then,  with  as  little  disturbance  of  the  diflferent 
intestines  as  possible,  observe  how  they  are  disposed  within  the  abdo- 
minal cavity. 

The  Intestines  (Plates  41  and  42).  The  intestinal  tube  begins  at  the 
pyloric  orifice  of  the  stomach,  and  it  "lerminates  on  the  surface  of  the 
body,  at  the  anus.  It  is  primarily  divided  into  small  and  large  intestines, 
and  each  of  these  is  naturally  or  arbitrarily  divided  into  segments. 

The  Small  Intestine  comprises  the  first  portion  of  the  tube,  and  in 
a  horse  of  medium  size  it  measures  about  seventy -two  feet  in  length.  As 
is  expressed  by  its  name,  it  is  of  smaller  calibre  than  the  large  intestine. 
Moreover,  it  is  distinguished  from  nearly  every  part  of  the  large  intestine 
l)y  having  a  smooth  and  regular  contour  when  distended.  The  first  two 
feet  of  the  tiibe  occupies  a  fixed  position,  and  is  termed  the  duodenum. 
It  received  this  name  because  in  man  its  length  is  about  equal  to  the 
breadth  of  twelve  fingers.  The  remainder  of  the  small  intestine  has  a 
comparatively  loose  mode  of  suspension ;  and  it  is  arbitrarily  divided 
into  jejunum  and  ileum,  the  former  succeeding  the  duodenum,  and 
measuring  about  thirty  feet,  the  latter  comprising  the  remainder  of  the 
tube — about  forty  feet.  These  terms  are  borrowed  from  human  anatomy, 
where  the  term  jejunum  was  applied  in  consequence  of  that  portion  of 
the  intestine  being  generally  found  empty  in  the  dead  body,  while  the 
ileum  was  so  designated  on  account  of  its  convoluted  disposition. 

The  Large  Intestine  is,  for  the  most  part,  of  vastly  greater  calibre  than 
the  small ;  and,  unlike  the  latter,  it  has  when  distended,  not  a  smooth, 
but  a  bosselated,  surfiice.  In  a  medium-sized  animal  it  is  about  twenty- 
five  feet  in  length.  It  is  subdivided — and  in  a  much  more  natural 
fixshion  than  the  small  intestine — into  ccecum,  colon,  and  rectum,  the 
colon  being  further  subdivided  into  double  and  single  colon. 

When  the  muscles  which  enclose  the  abdomen  below  and  on  each  side 
have  been  removed,  it  most  commonly  happens  that  only  the  large 
intestines  are  exposed,  and  consequently  their  examination  must  precede 
that  of  the  small  intestines. 

The  Cecum  is  the  first  of  the  large  intestines.     In  an  animal  of 


296  THE  ANATOMY  OF  THE  HORSE. 

medium  size  it  measures  about  three  feet  in  length,  and  when  moderately 
distended  it  has  a  capacity  of  about  four  gallons.  At  one  of  its  extremi- 
ties it  is  curved,  forming  what  is  termed  the  crook  of  the  cacum,  while 
the  opposite  extremity  tapers  to  a  blind  point,  from  which  the  bowel  is 
named.  The  bowel  has  a  puckered  appearance,  which  is  most  evident 
when  it  is  distended.  This  is  owing  to  the  longitudinal  muscular  fibres 
of  its  wall  being  not  unifonnly  distributed  as  they  are  in  the  small  intes- 
tine, but  collected  into  bands,  which  shorten  the  bowel  by  throwing  it 
into  folds.  The  terminal  portion  of  the  ileum  (small  intestine)  joins  the 
caecum  on  the  concave  side  of  the  crook,  and  a  few  inches  above  the 
point  of  conmaunication  is  the  orifice  by  which  alimentary  matters  arc 
passed  on  to  the  colon.  The  crook  of  the  caecum  is  fixed  in  the  right 
sublumbar  region  by  means  of  loose  cellular  tissue,  and  it  is  in  contact 
with  the  right  kidney  and  the  pancreas.  On  its  inner  side  it  adheres  by 
cellular  tissue  to  the  termination  of  the  doiible  colon,  and  the  duodenum 
passes  round  it  on  the  outer  side.  The  remaining  portion  of  the  bowel 
extends  do^\^l wards  and  forwards  through  the  right  hypochondriac  region, 
terminating  by  its  blind  point  in  the  epigastri\un.  The  first  portion  of 
the  large  colon,  which  lies  to  its  inner  side,  extends  in  the  same  direction, 
and  the  peritoneum  in  passing  from  the  one  bowel  to  the  other  forms  a 
fold  which  has  been  termed  the  meso-ccecum.  As  the  caecum  is  not 
adherent  to  the  abdominal  parietes  except  in  the  neighbourhood  of  its 
crook,  it  admits  of  some  displacement ;  and  the  student  must  therefore 
be  prepared  to  find  it  deviating  somewhat  from  the  course  just 
described. 

The  Double  or  Large  Colon.  This  bowel  is  termed  double  because 
when  taken  out  of  the  abdomen  it  is  aiTanged  in  the  form  of  two  paral- 
lel portions;  but  in  order  that  it  may  be  accommodated  within  the 
cavity,  it  has  again  to  be  doubled,  so  that  in  its  natural  disposition  it 
presents  four  portions,  which  receive  numerical  designations.  In  an 
animal  of  medium  size  its  length  is  about  ten  feet,  and  its  capacity 
about  sixteen  gallons.  It  is  p\ickered  like  the  caecum,  and  from  the 
same  cause. 

The  1st  division  of  the  bowel  begins  at  the  crook  of  the  caecum,  by 
an  orifice  of  commiuiication  which  is  comparatively  small.  It  extends 
downwards  and  forwards  through  the  right  hypochondriac  region,  b\ilg- 
ing  laterally  into  the  umbilical  region  ;  and  on  reaching  the  epigastrium, 
the  bowel  becomes  bent  on  itself,  forming  what  is  termed,  from  its  relation 
to  the  ensiform  cartilage  of  the  sternum,  the  suprastei-nal  flexure.  The 
angle  of  this  flexure  forms  the  point  of  separation  between  the  1st 
and  2nd  portions  of  the  double  colon. 

The  2nd  division,  beginning  at  the  suprasternal  flexure,  runs  back- 
wards on  the  left  side  of  the  abdomen,  occupying  the  hypochondriac, 
umbilical,  and  lumbar  regions;  and  on  approaching  the  entrance  of  the 


DISSECTION    OP    THE   ABDOMEN.  297 

pelvic  cavity,  the  bowel  forms  in  the  iliac  or  hypogastric  region  a  second 
flexure — the  pelvic  flexure,  the  angle  of  which  marks  the  point  of  separa- 
tion between  the  2nd  and  3rd  portions.  The  1st  and  2nd  portions  of  the 
double  colon  have  extensive  contact  with  the  abdominal  wall,  and  they 
conceal  the  other  two  divisions  of  the  bowel,  which  lie  above  them  (in  the 
natural  standing  posture). 

The  3rd  division,  beginning  at  the  pelvic  flexure,  extends  forwards 
along  the  left  side  of  the  abdomen,  through  the  same  areas  as  the  2nd 
portion,  being  closely  bound  to  it,  and  lying  immediately  above  it.  On 
reaching  the  epigastric  region,  a  third  flexure  is  formed,  in  contact  with 
the  diaphragm,  liver,  and  stomach,  and  from  these  relations  named  the 
diaphragmatic  or  gastro-hepatic  flexure.  This  will  be  brought  into  view 
by  grasping  and  pulling  backwards  the  siiprasternal  flexure,  above  which 
it  lies. 

The  4th  portion  begins  at  the  angle  of  the  diaphragmatic  flexure,  and 
passes  backwards  on  the  right  side  of  the  cavity,  lying  above  the  1st 
division,  and  closely  bound  to  it.  On  reaching  the  inner  side  of  the 
crook  of  the  caecum,  to  which  it  is  adherent,  it  suddenly  becomes  much 
reduced  in  calibre,  and  is  continued  as  the  small  or  floating  colon. 

The  pelvic  flexure  of  the  colon  should  now  be  seized  and  carried  for- 
wards, so  as  to  place  the  bowel  in  the  position  shown  in  Plate  41,  It  will 
now  be  observed  that  the  bowel  is  quite  unattached  except  at  its  beginning 
and  termination,  where  it  adheres  to  the  pancreas  and  the  crook  of  the 
csecum.  In  this  disposition  the  suprasternal  and  diaphragmatic  flexures 
are  obliterated,  and  the  1st  and  4th  portions  are  seen  to  be  closely 
adherent  to  one  another,  and,  in  like  manner,  the  2nd  and  3rd  portions, 
except  just  at  the  pelvic  flexure,  where,  in  the  angle  of  the  flexure,  a  small 
space  is  bridged  over  by  a  racket-shaped  piece  of  peritoneum.  It  will  be 
noticed  also  that  the  intestine  varies  greatly  in  calibre  at  different 
points.  Its  greatest  diameter  is  in  its  4th  portion,  and  its  smallest 
about  the  centre  of  the  3rd.  This  narrow  portion  of  the  intestine  is 
fui-ther  distinguished  from  the  rest  by  being  not  puckered,  but  plain, 
when  distended. 

The  Small  or  Floating  Colon  succeeds  the  double  colon.  It  is  much 
narroAver  than  that  bowel,  indeed  it  does  not  greatly  exceed  in  calibre 
the  small  intestine,  from  which,  however,  its  coils  are  readily  distin- 
guished by  their  puckered  appearance.  In  a  medium-sized  animal  it  is 
aboi\t  ten  feet  in  length.  It  is  disposed  within  the  abdomen  after  the 
manner  of  the  small  intestine,  being  suspended  at  the  free  edge  of  a  de- 
pendency of  the  peritoneum,  termed  the  vieso-colon  or  colic  mesentery.  It 
has  a  convoluted  disposition,  and  occupies  the  left  lumbar  and  iliac 
regions.  Its  last  coil  passes  into  the  pelvic  cavity,  and  is  continued  as 
the  rectum. 

The  Rectum  is  the  terminal  portion  of  the  intestine,  and  is  about  two 


298  THE  ANATOMY  OF  THE  HORSE. 

feet  ill  length.  It  derives  its  name  from  its  approximately  straight  course 
through  the  pelvic  cavity,  in  connection  with  which  it  will  be  more  fully 
described. 

Directions. — The  coils  of  the  jejunum  and  ileum  should  be  arranged 
in  the  left  flank  after  the  manner  of  Plate  41.  To  get  a  view  of  the 
duodenum,  the  ca3cum  should  be  thrown  across  the  abdomen,  with  its 
point  towards  the  left  side.  The  duodenum  will  then  be  seen  encircling 
the  crook  of  the  caecum  on  its  outer  side.  Should  the  large  intestine 
contain  much  ingesta,  that  should  be  evacuated  through  an  incision  across 
the  pelvic  flexure  of  the  double  colon  and  another  at  the  point  of  the 
caecum.  When  the  ingesta  has  been  expelled,  the  bowels  should  be 
moderately  inflated,  and  the  cut  ends  ligatured. 

The  Duodenum  (Plate  44)  is  the  first  segment  of  the  small  intestine. 
Its  length  is  about  two  feet,  but  it  cannot  be  very  well  seen  in  its 
entirety  at  this  stage  of  the  dissection.  It  begins  at  the  pyloric  aper- 
ture of  the  stomach,  Avhere  it  is  related  to  the  posterior  surface  of  the 
liver.  It  curves  upwards  and  backwards  across  the  lower  face  of  the 
right  kidney,  and  then  sweeping  round  the  crook  of  the  caecum  to  its 
outer  side,  it  crosses  the  spine  behind  the  anterior  mesenteric  artery,  and 
is  continued  as  the  jejunum.  It  is  maintained  in  position  by  a  narrow 
band  of  peritoneum,  and  in  this  fixity  of  position  it  is  distinguished  from 
the  rest  of  the  small  intestine. 

The  Jejunum  and  Ileum.  These  comprise  the  remaining  portion  of 
the  small  intestine,  of  which  about  thirty  feet  is  arbitrarily  appor- 
tioned to  the  former,  and  the  remainder  (about  forty  feet)  to  the  latter. 
They  are  arranged  in  the  form  of  numerous  coils,  which  occupy  the 
iliac,  umbilical,  and  hypogastric  regions.  The  coils  are  attached  to 
the  free  edge  of  a  fold  of  peritoneum  called  the  f/reat  mesentery ;  and 
inasmuch  as  this  mesentei-y  is  of  considerable  breadth,  they  may  move 
from  place  to  place  within  the  above-mentioned  areas.  When  distended, 
they  have  not  a  puckered,  but  a  smooth,  surface.  The  terminal  part  of 
the  ileum  joins  the  crook  of  the  caecum,  into  which  it  projects  for  a  little 
distance,  after  the  manner  of  a  tap  into  a  barrel;  and  at  the  point  of 
entrance  there  is  a  valvular  arrangement — the  ileo-cacal  valve,  to  pre- 
vent regurgitation  from  the  caecum  into  the  ileum. 

The  Peritoneum  is  the  lining  membrane  of  the  abdominal  and  pelvic 
cavities.  It  belongs  to  the  class  of  serous  membranes,  and,  like  all  such 
membranes,  it  consists  of  a  parietal  and  a  visceral  division,  these  being 
portions  of  one  great  sac.  The  pai-ietal  part  is  that  which  lines  the 
abdominal  walls,  or  parietes ;  the  visceral  part  invests  the  solid  and 
hollow  organs,  or  viscera,  of  the  abdominal  cavity.  In  virtue  of  this 
membrane,  all  the  free  surfaces  that  present  themselves  when  the 
abdominal  wall  is  removed,  have  a  smooth  and  shining  appearance. 
The  surface  of  the  membrane  is  covered  by  a  layer  of  endothelial  cells, 


DISSECTION    OF    THE    ABDOMEN.  299 

and  these  rest  upon  a  layer  of  vascular  connective-tissue.  The  object 
of  the  membrane  is  to  facilitate  the  movements  of  the  different  ab- 
dominal organs  on  each  other  and  on  the  walls  of  the  cavity,  and 
especially  to  facilitate  the  vermicular  or  peristaltic  movements  of  the 
intestines.  For  this  purpose  the  surface  of  the  membrane  is  kept  moist 
by  a  sparing  amount  of  serous  fluid,  which  gives  to  the  membrane  its 
glistening  aspect. 

To  trace  the  exact  disposition  of  the  peritoneum  in  the  horse  is  ver}' 
difficult,  in  consequence  of  the  vmwieldy  character  of  the  intestines. 
When  the  student  has  the  opportunity  he  should  examine  the  mem- 
brane in  a  foal,  in  which  the  different  organs  can  be  manipulated  with 
ease. 

The  parietal  and  visceral  peritoneum,  as  has  already  been  stated, 
form  portions  of  one  great  sac,  and  the  various  abdominal  viscera  are 
external  to  this  sac.  The  sac  of  the  peritoneum,  it  must  be  observed, 
encloses  not  an  actual,  but  merely  a  potential,  cavity ;  the  inner  surface 
of  every  portion  of  the  sac  being  in  contact  with  the  same  surface  of 
another  portion.  To  facilitate  the  understanding  of  this,  let  the  student 
imagine  the  cavity  of  the  abdomen  (including  the  pelvis)  as  having  its 
natural  form,  but  deprived  of  all  its  contents,  and  completely  lined  by 
peritoneum,  which,  for  simplicity's  sake,  he  may  suppose  to  be  elastic. 
The  continuity  of  the  membrane,  and  the  fact  that  it  formed  a  close 
sac  would  then  be  apparent.  Now  let  him  imagine  a  simple  tube  of 
intestine  extending  between  this  membrane  and  the  spinal  column,  that 
is,  outside  the  serous  sac.  Conceive  next  this  tube  of  intestine  let 
gradually  down,  until  it  extends  thi'ough  the  cavity  about  its  centre. 
In  this  descent  the  intestine  would  first  surround  itself  with  peritoneum ; 
and  then,  as  it  sank  farther,  it  would  stretch  the  membrane  so  as  to 
form  a  kind  of  sling  passing  upwards  to  the  point  from  which  it  started. 
The  membrane  would  now  have  lost  its  simplicity,  for  it  would  have  a 
parietal  division  continuing  to  line  the  abdominal  walls,  and  a  visceral 
portion  surrounding  the  tube  of  intestine.  Moreover,  these  two  portions 
would  be  continuous  with  each  other  along  the  sling-like  portion  sus- 
pending the  tube.  Lastly,  imagine  the  tube  of  intestine  to  grow  and 
branch,  so  as  to  completely  fill  up  the  abdominal  cavity,  and 
obliterate  the  space  between  the  parietal  and  visceral  peritoneum.  This, 
of  course,  would  not  destroy  the  continuity  of  the  serous  sac,  although  it 
would  complicate  it  so  that  its  continuity  would  be  difficult  to  trace. 

All  the  organs,  then,  that  actually  project  into  the  abdominal  cavity 
get  a  more  or  less  complete  investment  of  visceral  peritoneum  ;  and,  in 
the  case  of  each  organ,  this  visceral  covering  is  traceable  on  to  a  neigh- 
bouring organ,  or  on  to  the  walls  of  the  abdomen.  Where  organs  are 
contiguous  to  each  other  or  to  the  abdominal  parietes,  the  peritoneum 
may  pass  directly  from  the  one  organ  to  another  or  to  the  abdominal 


300  THE  ANATOMY  OF  THE  HORSE. 

parietes ;  but,  at  otlier  times,  the  connection  between  the  parietal  and 
visceral  peritoneum  is  traceable  along  bands  or  folds  analogous  to  the 
sling-like  membrane  that  was  formed  in  the  imaginary  case.  These 
folds  constitute  the  various  mesenteries,  omenta,  and  peritoneal  liga- 
ments that  will  hereafter  be  described. 

Although  there  is  but  a  single  peritoneal  sac,  this  sac  is  so  disposed 
that  it  forms  two  compartments,  termed  respectively  the  greater  and 
lesser  cavities  of  the  peritoneum,  the  latter  being  also  known  as  the 
cavity  of  Winslow.  The  greater  cavity  is  that  which  is  exposed  when 
the  inferior  wall  of  the  abdomen  is  removed,  the  lesser  cavity  is  situated 
behind  the  stomach,  and  is  separated  from  the  greater  cavity  mainly  by 
the  omentum. 

The  Great  or  Gastro-colic  Omentum. — Passing  backwards  among  the 
intestines,  on  the  left  side  of  the  abdomen,  there  will  have  been  noticed 
a  large  lace-like  membrane,  which  is  the  fjreat  omentum^  epiiilorm,  or  web. 
In  order  to  examine  its  connection,  the  caecum  and  double  colon  should 
be  thrown  backwards  over  tlie  right  flank,  and  the  coils  of  the  single 
colon  arranged  over  the  left  flank.  The  coils  of  small  intestine  should 
at  the  same  time  be  gathered  backwards  and  to  the  right.  The  omen- 
tum is  composed  of  two  layers  of  peritoneum,  Avhich  include  between 
them  vessels,  and  a  vaiying  quantity  of  fat.  This  fat  is  deposited 
mainly  along  the  course  of  the  vessels,  leaving,  except  in  obese  subjects, 
inters'eniug  transparent  areas  that  are  free  from  fat  ;  and  it  is  from  this 
arrangement  that  the  membrane  possesses  a  lace-like  appearance.  The 
two  layers  of  the  omentum  may  be  distinguished  as  superficial  and 
deep. 

When  the  superficial  layer  is  traced  backwards,  it  is  seen  to  pass  on 
to  the  terminal  part  of  the  double  colon  (4th  part)  and  initial  part  oi' 
the  single  colon,  covering  the  posterior  aspect  of  these  where  they  extend 
across  the  roof  of  the  abdominal  cavity.  Behind  these  it  passes  back- 
wards along  the  roof  of  the  abdominal  cavity,  from  which  it  descends  to 
envelop  the  small  intestine,  forming  the  great  mesentery,  and  the  float- 
ing colon,  forming  the  colic  mesentery.  To  the  right,  again,  it  passes 
directly  on  to  the  caicum  and  the  double  colon ;  and  after  enveloping 
these  intestines,  it  returns  to  the  abdominal  wall,  to  pursue  its  back- 
ward course  to  the  pelvis.  When  followed  forwards,  the  superficial 
layer  reaches  the  convex  curvature  of  the  stomach,  and  the  initial  dila- 
tation of  the  duodenum;  and  it  passes  over  the  anterior  surfaces  of 
these  organs  as  visceral  peritoneum.  Passing  off"  the  duodenum  and 
stomach,  it  next  forms  the  anterior  layer  of  the  gastro-hepatic  omen- 
tum, and  thus  reaches  the  posterior  surface  of  the  liver  at  the  portal 
fissure.  From  tliat  point  it  descends  over  the  posterior  surface  of  the 
liver  as  visceral  peritoneum,  and  turns  round  the  inferior  edge  of  the 
gland  to  gain  its  diaphragmatic  surface.     It  ascends  on  this  surface  ; 


DISSECTION    OF    THE    ABDOMEN.  301 

and  where  the  liver  and  diaphragm  are  united,  it  passes  from  the  former 
to  the  latter,  on  which  it  descends  to  the  inferior  wall  of  the  abdomen. 
Along  this  it  passes  until  it  entei's  the  pelvis,  where  it  becomes  con- 
tinuous with  the  same  layer  already  followed  backwards  along  the  roof 
of  the  abdomen.  In  the  male  it  is  to  be  observed  that  the  parietal 
peritoneum  of  the  abdominal  floor  passes  into  the  inguinal  canal,  and 
forms  the  tunica  vaginalis  of  the  testicle,  the  sac  of  which  is  a  simple 
diverticulum  of  the  great  peritoneal  sac.  Returning  again  to  the 
omentum,  it  will  be  noticed  that  its  superficial  layer,  towards  the  left 
side,  in  passing  forwards  to  gain  the  convex  curvature  of  the  stomach, 
encounters  the  spleen.  Passing  round  that  organ,  it  gives  to  it  a 
visceral  covering,  and  then  continues  its  course  to  the  stomach.  The 
portion  of  omentum  between  the  spleen  and  the  left  sac  of  the  stomach 
is  termed  the  gastro-splenic  omentum. 

Now  make  a  transverse  opening  about  the  centre  of  the  great  omen- 
tum, and  introduce  the  hand  through  the  opening.  The  hand  is  now  in 
what  is  termed  the  cavity  of  Winsloiv,  and  the  deep  layer  of  the  omen- 
tum is  exposed.  When  this  layer  is  traced  forwards,  it  is  seen  to  reach 
the  convex  curvature  of  the  stomach,  where,  separating  from  the  super- 
ficial layer,  it  passes  over  the  posterior  surface  of  the  stomach,  and 
initial  dilatation  of  the  duodenum.  From  these,  again,  it  passes  as  the 
posterior  layer  of  the  gastro-hepatic  omentum,  and  reaches  the  liver  at 
the  portal  fissure.  There  it  separates  from  the  other  layer  of  the  gastro- 
hepatic  omentum,  and  ascends  on  the  liver.  It  turns  round  the 
superior  edge  of  the  gland,  and  passes  from  its  anterior  face  to  the  dia- 
phragm, on  which  it  ascends  to  the  spine.  The  deep  layer  of  the  omen- 
tum is  now  to  be  followed  in  the  backward  direction.  It  is  seen  to 
reach  the  terminal  part  of  the  double  colon,  and  the  initial  part  of  the 
single  colon ;  and,  separating  there  from  the  superficial  layer,  it  passes 
over  the  anterior  aspect  of  these  portions  of  intestine,  and  is  reflected 
forwards  on  the  under  svirface  of  the  pancreas.  It  turns  round  the 
anterior  edge  of  that  gland,  covers  for  a  little  distance  its  upper  face, 
and  then  passes  on  to  the  spine,  where  it  meets  the  same  layer  advanc- 
ing in  the  opposite  direction.  It  is  thus  seen  that  th6  deep  layer  of  the 
omentum,  when  traced  in  the  antero-posterior  direction,  forms  a  con- 
tinuous layer ;  and  at  first  sight  it  does  not  appear  to  be  continuous 
with  the  remainder  of  the  peritoneum.  As  already  stated,  however,  the 
peritoneum  forms  a  single  sac,  and  the  before-mentioned  layer  is  con- 
tinuous with  the  remainder  of  the  serous  membrane  at  a  narrow  opening- 
termed  the  foramen  of  Winsloiv.  To  find  this  opening,  pass  the  dorsal 
aspect  of  the  left  forefinger  along  the  posterior  surface  of  the  lobulus 
caudatus  of  the  livei",  close  to  the  spine ;  and  insinuate  the  point  of  the 
finger  onwards  towards  the  left  (of  the  subject).  At  the  same  time  pass 
the  right  hand  up  to  the  spine  in  the  cavity  of  Winslow,  and  insinuate  the 


302  THE    AXATOMV    OF    THE    HOR«E. 

forefinger  towards  the  right,  above  and  beliind  the  pylorus.  The  tips  of 
the  forefingers  of  opposite  hands  can  thus  be  made  to  meet,  showing  tlie 
continuity  of  the  larger  sac  of  the  peritoneum,  in  which  the  left  hand  is, 
with  the  smaller  sac,  or  cavity  of  Winslow,  in  which  the  right  hand  is. 
Perhaps  the  simplest  way  to  get  an  understanding  of  the  relationship  of 
the  two  cavities,  is  to  imagine  the  deep  layer  of  the  omentum  to  be 
suppressed.  In  that  condition,  the  anterior  aspect  of  the  double  and 
single  colon  at  their  point  of  junction,  the  pancreas,  the  posterior 
surface  of  the  stomach  and  initial  dilatation  of  the  duodenum,  the  upper 
parts  of  the  liver  and  diaphragm,  and  the  roof  of  the  abdomen  for  a 
short  space  behind  the  hiatus  aorticus  would  be  without  a  serous  cover- 
ing. It  may  be  supposed  that  to  supply  this  deficiency,  a  pouch  of  the 
great  sac  of  peritoneum  has  to  be  made.  This  pouch  is  made  at  the 
foramen  of  Winslow,  the  peritoneiim  being  there  thrust  outwards 
towards  the  right,  and  expanded  xmtil  it  forms  Avhat  has  already  been 
traced  as  the  deep  layer  of  the  omentiim.  The  foramen  will  be  observed 
to  have  the  following  boundaries  : — the  base  of  the  lobulus  caudatus  in 
front,  the  4th  part  of  the  double  colon  behind,  the  free  edge  of  the 
gastro-hepatic  omentum  below,  and  the  posterior  vena  cava  and  right 
pillar  of  the  diaphragm  above. 

The  Great  Mesentery  is  the  membrane  that  suspends  the  small  intes- 
tine. Like  the  omentum,  it  is  composed  of  two  layers  of  peritoneum. 
These  layers  leave  the  spine  at  the  root  of  the  anterior  mesenteric 
artery,  being  thei'e  continuous  with  the  parietal  peritoneum ;  and  they 
descend,  one  on  each  side  of  the  branches  of  that  artery,  until  they 
reach  the  intestine.  At  the  concave  edge  of  the  bowel  the  two  layers 
separate ;  and  after  encircling  the  tube  as  visceral  peritoneum,  they 
meet  and  become  continuous  at  its  convex  or  free  border.  Where  the 
mesentery  suspends  the  first  part  of  the  jejunum,  it  is  continuous  with 
the  peritoneal  freenum  of  the  duodenum ;  and  at  its  ojjposite  extremity, 
where  it  envelops  the  termination  of  the  ileum,  it  passes  on  to  the  ctecum. 
At  the  latter  point  it  will  be  observed  that  the  two  layers  of  mesentery 
do  not  become  continuous  around  the  convex  border  of  the  ileum,  but 
are  prolonged  beyond  that,  so  that  the  terminal  portion  of  the  small 
nitestine  is  included  in  the  mesentery  some  distance  from  its  free  edge. 

The  Colic  Mesentery. — This  is  the  membrane  that  suspends  the  single 
or  floating  colon.  It  is  composed  of  two  layers  of  peritoneum,  which 
leave  the  roof  of  the  abdomen  along  a  line  extending  from  the  root  of 
the  anterior  mesenteric  artery  to  the  inlet  of  the  pelvis.  These  two 
layers  include  between  them  the  posterior  mesenteric  artery  and  its 
branches ;  and  after  enveloping  the  single  colon,  they  become  continuous 
at  its  free  edge.  At  its  anterior  extremity  the  colic  mesentery  is  con- 
tinuous with  the  great  omentum  and  with  the  great  mesenter}',  and  at 
the  pelvic  inlet  it  is  continuous  with  the  meso-rectum. 


DISSECTION    OF    THE    ABDOMEN.  303 

The  Uterine  Broad  Ligaments. — These  are  the  double  peritoneal  folds 
that  suspend  the  uterus,  ovaries,  and  Fallopian  tubes.  Each  ligament 
leaves  the  roof  of  the  abdomen  in  the  lumbar  region,  and  descends  to 
the  concave  edge  of  the  cornu,  and  to  the  side  of  the  upper  face  of  the 
body,  of  the  uterus.  At  these  points  the  layers  of  the  ligament  sepa- 
rate, and  pass  on  to  the  uterus  as  its  visceral  covering.  The  ligaments 
are  widely  apart  in  front ;  but  as  they  are  traced  backwards,  they 
become  narrower  and  nearer  to  each  other.  The  Fallopian  tube  is  sus- 
tained between  the  two  layers  of  each  ligament  at  its  anterior  edge,  and 
here  the  fimbriated  extremity  of  the  tube  opens  into  the  sac  of  the  peri- 
toneum. In  the  female,  therefore,  the  peritoneum  does  not  form  a  shut 
sac.  Stretching  between  the  ovary  and  the  uterine  cornu  is  a  cord  of 
non-striped  nuiscular  tissue — the  ligament  of  the  ovary — which  forms 
the  free  edge  of  a  small  secondary  fold  of  peritoneum.  This  forms  with 
the  adjacent  part  of  the  broad  ligament  a  pocket-like  cavity.  On  the 
outer  side  of  the  broad  ligament  another  secondary  fold  extends  as  far 
as  the  internal  abdominal  ring,  and  contains  a  layer  of  non-striped 
muscular  tissue  corresponding  to  the  round  ligament  of  the  human 
uterus.  Besides  some  scattered  fibres  of  non-striped  muscle,  the  layers 
of  the  broad  ligament  include  between  them  the  uterine  and  ovarian 
vessels  and  nerves. 

The  other  peritoneal  ligaments  will  be  described  in  connection  with 
the  organs  to  which  they  belong. 

Peritoneal  Pockets.  The  peritoneum,  in  passing  from  one  organ  to 
another,  forms  several  remarkable  pockets,  one  of  which  has  been  men- 
tioned above  in  connection  with  the  ligament  of  the  ovary.  The  exact 
position  of  the  others  will  now  be  indicated.  So  far  as  I  am  aware, 
these  have  not  hitherto  been  described.  Nevertheless,  they  possess 
considerable  interest,  since,  in  the  human  subject,  a  coil  of  intestine  has 
been  known  to  become  incarcerated  in  a  similar  pocket  of  peritoneum. 

1.  The  entrance  to  the  first  of  these  pockets  will  be  found  imme- 
diately in  front  of  the  base  of  the  lobulus  caudatus,  which  separates  it 
from  the  foramen  of  Winslow.  It  is  bounded  by  the  anterior  end  of  the 
right  kidney,  and  by  the  lobulus  caudatus  and  upper  part  of  the  right 
lobe  of  the  liver.  It  extends  inwards  to  near  the  spine  between  the 
diaphragm  and  the  upper  part  of  the  right  lobe  of  the  liver. 

2.  Another  pocket  will  be  found  a  little  to  the  left  of  the  root  of  the 
anterior  mesenteric  artery,  the  entrance  to  it  being  on  the  anterior 
surface  of  the  mesentery  suspending  the  first  few  inches  of  the  jejunum. 
The  pocket  is  bounded  in  part  by  this  piece  of  mesentery,  and  in  part 
by  a  peritoneal  fold  passing  between  the  jejunum  and  the  first  part  of 
the  single  colon. 

3.  Other  two  pockets  will  be  found  at  the  termination  of  the  small 
intestine.      Turn  the  point  of  the  crecum  backwards  and  to  the  right. 


304  TUE  AX  ATOM  V  OF  THE  HORSE. 

and  pull  upon  the  terminal  part  of  the  jejunum.  On  each  side  of  the 
point  at  which  the  latter  perforates  the  csecal  crook,  there  will  be  found 
a  recess,  the  posterior  (in  this  position)  being  the  deeper. 

4.  Another  considerable  pocket  will  readily  be  found  on  the  concave 
side  of  the  Cfscal  crook,  being  formed  by  the  peritoneum  in  passing 
between  the  caecum  and  the  beginning  of  the  double  colon. 

5.  Another  but  much  smaller  pocket  will  be  found  in  the  cavity  of 
Winslow,  above  and  in  front  of  the  first  few  inches  of  the  single  colon. 

Directions. — For  the  display'  of  the  mesenteric  vessels  and  the  sympa- 
thetic nerve,  the  intestines  should  first  be  disposed  after  the  manner  of 
Plate  41.  When  well  injected,  the  arteries  require  but  little  dissection, 
and  they  are  closely  accompanied  by  the  veins  and  nerves.  The  arteries 
of  the  caecum  and  colon  should  be  taken  where  most  conspicuous,  and 
traced  in  both  directions.  Each  of  these  vessels  must  be  carefully  dis- 
sected up  to  its  point  of  origin,  but  only  two  or  three  of  the  arteries  of 
the  small  intestine  need  be  fully  dissected.  The  whole  intestinal  tube 
with  the  exception  of  a  shoi't  piece  of  the  duodenum  next  the  stomach, 
is  supplied  by  the  anterior  and  posterior  mesenteric  arteries,  which  are 
branches  of  the  abdominal  aorta.  The  first  supplies  the  whole  of  the 
small  intestine  except  the  piece  of  duodenum  specified;  and  it  also  sup- 
plies the  caecum,  the  large  colon,  and  a  few  inches  of  the  beginning  of 
the  small  colon.  The  remainder  of  the  small  colon,  and  the  rectum  are 
supplied  by  the  posterior  mesentei'ic  artery. 

The  Anterior  Mesenteric  Artery  (Plate  41)  comes  off  from  the 
inferior  aspect  of  the  aorta  at  the  1st  lumbar  vertebra.  It  is  only  about 
an  inch  and  a  half  in  length,  but  it  has  a  large  calibre ;  and  in  old  horses 
it  often  shews  aneurismal  dilatation.  It  divides  into  three  terminal 
branches,  which  from  their  direction  ai'e  distinguished  as  left,  fight, 
and  anterior.  The  left  distributes  its  branches  to  the  whole  of  the  small 
intestine  except  a  few  inches  at  the  beginning  of  the  duodenum  and 
about  two  feet  at  the  end  of  the  ileum;  the  right  su])plies  the  terminal 
portion  of  the  ileum,  the  entire  cfccum,  and  the  double  colon  as  f\xr  as 
the  pelvic  flexure;  and  the  anterior  is  distributed  to  the  double  colon 
Ijeyond  the  pelvic  flexure,  and  to  the  first  few  inches  of  the  single  colon. 
It  is  an  assistance  to  the  memory  to  study  the  different  branches  in  the 
order  of  their  distril)ution  to  the  intestine,  taking  first  those  that  supply 
the  most  anterior  segment  of  the  tube. 

1.  The  Left  Branch  of  the  anterior  mesenteric  artery  is  no  sooner 
detached  than  it  splits  up  into  about  fifteen  or  twenty  arteries,  which 
pass  between  the  laj'ers  of  the  mesentery  to  supply  the  small  intestine. 
Indeed,  the  left  branch  can  scarcely  be  said  to  exist,  for  these  arteries  of 
the  small  intestine  seem  to  spring  from  a  common  point  of  tlie  anterior 
mesenteric  trunk.  As  each  artery  appi'oaches  the  intestine,  it  bifurcates, 
each  branch  inosculating  with  the  corresponding  branch  of  an  adjacent 


DISSECTION    OF    THE    ABDOMEN.  305 

artery  to  form  an  arch.  From  the  convexity  of  these  arches  smaller 
vessels  pass  to  each  side  of  the  intestine,  and  anastomose  ronnd  it.  At 
the  anterior  part  of  the  tube  two  sets  of  superposed  arches  are  formed 
before  the  ultimate  vessels  to  the  intestine  are  detached.  The  branch 
which  is  most  anterior  in  point  of  distribution  anastomoses  with  the 
duodenal  branch  of  the  cocliac  axis,  while  the  one  which  is  most  pos- 
terior anastomoses  with  the  ileo-ceecal  artery  from  the  right  branch  of 
the  anterior  mesenteric. 

2.  The  Right  Branch  of  the  anterior  mesenteric  artery  divides  into 
four  vessels,  viz.,  the  ileo-csecal,  the  superior  csecal,  the  inferior  ceecal, 
and  the  direct  colic  arteries. 

a.  The  Ileo-ccecal  Artery  (Plate  41,  for  ilio-ccecal  read  ileo-ccecal)  sup- 
plies the  terminal  poilion  of  the  ileum  (about  two  feet  in  length),  and 
inosculates  with  the  last  of  the  arteries  from  the  left  branch. 

b.  The  Superior  C cecal  Artery,  in  the  present  inverted  position  of  the 
intestines,  passes  beneath  the  termination  of  the  ileum  to  run  along  one 
of  the  longitudinal  muscular  bands  of  the  caecum.  It  sometimes  gives 
off  the  ileo-csecal  artery  as  a  collatei-al  branch,  and  at  the  point  of  the 
caecum  it  anastomoses  with  the  next  vessel.  It  gives  off  branches  right 
and  left  to  the  walls  of  the  csecum. 

c.  The  Inferior  Ccecal  Artery,  in  the  present  position  of  parts,  passes 
above  the  termination  of  the  ileum  to  run  along  another  of  the  muscular 
bands  of  the  caecum.  Besides  collateral  branches  to  the  main  portion  of 
the  bowel,  it  gives  off  the  artery  of  the  arch,  which  follows  the  concavity 
of  the  csecal  crook  and  terminates  on  the  beginning  of  the  double 
colon. 

d.  The  Direct  or  Eight  Colic  Artery. — This  is  a  large  vessel,  receiving 
the  first  of  these  designations  because  the  course  of  its  blood  stream  is 
the  same  as  that  of  the  alimentary  matters  in  the  bowel.  It  supplies, 
by  right  and  left  collateral  branches,  the  1st  and  2nd  portions  of  the  double 
colon,  and  anastomoses  at  the  pelvic  flexure  Avith  the  retrograde  colic 
artery. 

3.  The  Anterior  Branch  of  the  anterior  mesenteric  artery  divides  after 
a  very  short  coiu-se  into  two  vessels  of  unequal  size,  viz.,  the  retrograde 
colic  artery  and  the  first  artery  of  the  small  colon. 

a.  The  Retrograde  or  Left  Colic  Artery,  much  the  larger  of  the  two, 
supplies  successively  the  4th  and  3rd  portions  of  the  double  colon,  run- 
ning parallel  to  the  direct  colic  artery,  but  carrying  its  blood  in  a  direc- 
tion counter  to  the  course  of  the  alimentary  matters  in  the  intestine. 

h.  The  First  Artery  of  the  Small  Colon  supplies  a  short  piece  at  the 
beginning  of  that  bowel.  It  is  included  between  the  layers  of  the  colic 
mesentery,  and  auostomoses  with  the  first  branch  of  the  posterior 
mesenteric  artery. 

Directions. — To  display  the  posterior  mesenteric  artery,  the  small 

X 


30G  THE  ANATOMY  OP  THE  HORSE. 

colon    must   be  spread  out   over   the  left  flank  after  the  manner  of 
Plate  42. 

The  P0.STER10R  Mesenteric  Artery  is  a  much  smaller  vessel  than  the 
anterior.  It  is  a  branch  of  the  abdominal  aorta,  from  which  it  is  given 
off  at  the  -ith  lumbar  vertebra.  It  passes  in  a  curved  direction  between 
the  laj'ers  of  the  colic  mesentery  and  meso-rectum,  and  terminates  near 
the  anus  in  vessels  which  supply  the  end  of  the  intestinal  tube. 
From  the  convexity  of  its  curve,  which  is  directed  downwards,  about 
twelve  or  fourteen  branches  pass  to  supply  the  small  colon  (except  a  few 
inches  at  its  beginning)  and  the  rectum.  The  branches  which  supplj' 
the  first  half  of  the  small  colon  divide  and  form  arches  by  anastomosis 
in  the  mesentery,  close  to  the  bowel ;  but  the  more  posterior  branches 
do  not  anastomose  until  they  perforate  the  intestinal  wall. 

The  Intestinal  Veins. — The  blood  which  is  brought  to  the  intestines  by 
the  arteries  just  considered  is  carried  away  by  vessels  belonging  to 
the  portal  system.  These  veins  for  the  most  part  i-un  in  close  company 
witli  the  arteries,  and  receive  the  same  names. 

The  Anterior  Mesenteric  Vein  is  a  veiy  large  vessel  having  tribut- 
aries which  coiTespond  almost  exactly  to  the  divisions  of  the  artery  of 
the  same  name.  It  joins  the  splenic  and  posterior  mesenteric  veins  to 
constitute  the  vena  portso. 

Tlie  Posterior  Mesenteric  Vein  has  its  roots  in  the  hsemorrhoidal 
veins  around  the  termination  of  the  rectum,  which  veins,  on  the  other 
hand,  communicate  with  the  internal  pudic  vein.  After  receiving  blood 
from  the  walls  of  the  rectum  and  small  colon,  the  posterior  mesenteric 
vein  forms  by  union  with  the  splenic  a  very  short  trunk  which  joins 
the  anterior  mesenteric  to  form  the  vena  portte. 

Lymphatic  Vessels  of  the  Intestine. — In  an  ordinary  dissecting-room 
subject  the  l^nnphatic  vessels  will  not  be  visible  imless  the  animal  is 
emaciated  and  has  been  killed  shortly  after  a  meal,  in  which  case  the 
mesenteric  vessels  may  be  seen  without  dissection.  They  will  be  recog- 
nised as  vessels  with  very  thin  walls  and  milky  contents,  coui-sing 
between  the  layers  of  the  mesentery,  from  the  intestine  towards  the 
anterior  mesenteric  artery.  The  Ijonpathic  vessels  of  the  small  intestine 
are  called  lacteals. 

The  Lymphatic  Glands  of  the  Intestine  are  very  numerous.  Those 
of  the  small  intestine  are  chiefly  aggregated  in  the  form  of  a  cluster  of 
about  tliirty  included  between  the  layers  of  the  mesentery,  near  the 
anterior  mesenteric  artery ;  but  a  number  are  placed  lower  down  in  the 
mesentery,  along  the  course  of  the  ileo-caecal  artery.  The  glands  of  the 
caecum  are  distributed  in  the  form  of  two  chains  along  the  track  of  the 
superior  and  inferior  cajcal  arteries,  and  numerous  glands  are  similarly 
placed  on  the  colon  along  the  course  of  the  direct  and  retrograde  colic 
arteries.     Those  of  the  small  colon  and  rectum  are,  for  the  most  part, 


DISSECTION    OF    THE    ABDOMEN.  307 

placed  on  the  Avail  of  the  bowel,  at  the  edge  of  the  mesentery  ;  but  a  few- 
are  included  between  the  layers  of  the  colic  mesentery.  The  lacteals 
from  the  small  intestine  and  the  lymphatic  vessels  from  the  large  intes- 
tine traverse  these  various  groups  of  glands  on  their  course  towards  the 
receptaculum  chyli. 

The  Sympathetic  Nebve.  This  nerve  forms  on  the  aorta,  in  front  of 
the  anterior  mesenteric  artery,  a  great  network  termed  the  Solar  plexus. 
The  solar  plexus  is  at  present  concealed  by  the  pancreas,  but  the  student 
has  to  notice  the  anterior  and  posterior  mesenteric  plexuses,  which  are 
wholly  or  in  part  derived  from  it. 

The  Anterior  Mesenteric  Plexus  comprises  numerous  nerves  already 
met  in  dissecting  the  branches  of  the  anterior  mesenteric  arteiy.  The 
nerves  interlace  around  the  arteries,  and  pass  with  them  to  gain  the 
bowel,  where  they  further  interlace  before  penetrating  its  wall. 

The  Posterior  Mesenteric  Plexus. — The  branches  of  this  plexus  run  in 
company  with  the  divisions  of  the  artery  of  the  same  name.  Its  nerves 
are  derived  in  part  from  the  aortic  plexus,  which  is  a  backward  continua- 
tion of  the  solar  plexus,  and  in  part  from  roots  furnished  by  the  lumbar 
cord  of  the  sympathetic.  The  ultimate  branches  are  distributed  in  the 
wall  of  the  small  colon  and  rectum. 

Directions. — The  intestinal  mass  is  now  to  be  removed  in  the  following 
manner.  The  ropes  must  be  unfastened  from  the  left  limbs  of  the  animal, 
while  those  on  the  right  limbs  are  to  be  lengthened  until  the  subject 
inclines  considerably  over  to  the  left  side.  Two  ligatures  a  few  inches 
apart  are  to  be  passed  round  the  duodenum  where  it  encircles  the  crook 
of  the  caecum,  and  the  bowel  is  then  to  be  cut  across  between  the  liga- 
tures, the  object  of  which  is  to  keep  the  contents  from  escaping.  Where 
the  small  colon  joins  the  rectum,  at  the  entrance  to  the  pelvis,  the  bowel 
is  to  be  served  in  the  same  Avay,  and  the  colic  mesentery  is  to  be  cut 
along  its  point  of  origin  at  the  spine.  Both  large  and  small  intestines 
are  then  to  be  throAvii  as  far  as  possible  oiitwards  over  the  left  flank. 
The  next  step  must  be  to  take  the  scalpel  and  carefully  sever  the  con- 
nective-tissue adhesions  between  the  caecal  crook  and  colon  on  the  one 
hand,  and  the  sublumbar  region  and  pancreas  on  the  other.  In  doing 
this,  the  dissector  must  cut  close  to  the  wall  of  the  bowel,  and  take 
especial  care  not  to  take  away  any  portion  of  the  pancreas,  which  will  be 
recognised  by  its  dark  colour.  The  operation  will  be  favoured  by  the 
weight  of  the  intestines,  which  tends  to  tear  these  connections.  When 
the  caecum  and  colon  have  been  freed,  it  will  be  found  that  strong  resist- 
ance to  the  removal  of  the  intestines  is  still  offered  by  the  mesentery,  or 
rathei-,  by  its  included  vessels.  These  must  therefore  be  cut  near  the 
spine,  and  the  entire  mass  will  then  slip  over  the  left  side,  the  omentum 
being  cut  or  torn  from  its  attachment  to  the  colon.  The  intestines 
should  now  be  spread  out  on  a  table ;  and  when  the  student  has  refreshed 


308  THE  ANATOMY  OF  THE  HORSE. 

his  memory  regarding  their  form  and  connections  with  one  another,  he 
must  proceed  to  examine  their  structure.  This  should  be  done  by  taking 
a  short  piece  of  the  gut,  slitting  it  up,  and  pinning  it  with  its  mucous 
surface  downwards  on  a  block  of  wood. 

Structure  of  the  Small  Intestine.  The  wall  of  the  bowel  is  made 
up  of  four  layers,  viz.,  serous,  muscular,  submucous,  and  mucous. 

1.  The  Serons  Layer,  the  most  external,  is  a  part  of  the  visceral 
peritoneum.  It  reaches  the  bowel  by  the  mesentery,  whose  two  layers 
separate  at  the  concave  border  of  the  intestine,  and  pass  round  each  side 
to  meet  and  become  continuous  on  its  convex  or  free  border.  It  is 
closely  adherent  to  the  subjacent  muscular  layer,  which  it  completely 
covers  except  at  the  line  of  separation  of  the  two  layers  of  the  mesentery, 
where  the  vessels  enter.     It  must  be  stripped  off  to  expose  the  next  coat. 

2.  The  Muscular  Goat  is  made  up  of  two  distinct  sets  of  fibres: 
1.  Lono-itudinal  fibres,  which  are  most  extenial,  and  form  a  thin  layer 
uniformly  spread  along  the  wall.  2.  Circular  fibres,  thicker  than  the 
preceding,  and  also  spread  over  every  part  of  the  wall.  These  fibres  are 
of  the  pale,  non-striated  variety. 

3.  The  Submucous  Coat  is  composed  of  loose  areolar  tissue  uniting 
the  muscular  and  mucous  layers.  In  the  duodenum  it  contains  the 
o-lands  of  Brunner,  which  have  the  racemose  type  of  structure,  and  arc 
about  the  size  of  a  hemp  seed.  Their  ducts  pass  through  the  mucous 
membrane,  and  open  on  its  free  surface. 

Directions. — A  few  feet  of  the  jejunum  and  about  the  same  length  of 
the  ileum  should  be  taken  and  slit  up  along  the  line  of  attachment  of 
the  mesentery.  After  the  pieces  have  been  gently  washed,  they  should 
be  spread  on  a  flat  surface  with  the  peritoneal  coat  downwards. 

4.  The  Mucous  Memh-ane  forms  an  inner  lining  to  the  intestine.  It  is 
a  soft,  velvety -looking  membrane  which,  when  healthy  and  fresh,  has  a 
pinkish-yellow  colour.  When  a  piece  of  intestine  is  floated  in  water,  the 
mucous  membrane  is  seen  to  be  studded  with  short,  thread-like  projec- 
tions, to  Avhich  the  velvety  appearance  of  the  membrane  is  due.  These 
are  the  intestinal  villi.  Each  of  them  may  be  regarded  as  an  upheaval 
of  the  mucous  membrane,  containing  in  its  interior  microscoj^ic  blood 
and  lymph  vessels,  some  non-striped  muscular  fibres,  and  a  framework 
of  lymphoid  tissue.  The  villi  are  important  agents  in  the  absorption 
of  nutrient  particles  from  the  contents  of  the  bowel.  They  are  found 
throuo-hout  the  whole  of  the  small  intestine,  but  are  more  numerous  in 
the  jejunum  than  in  the  ileum.  The  free  surfiice  of  the  mucous  mem- 
brane, including  the  villi,  is  formed  by  a  single  layer  of  columnar  epi- 
thelium with  goblet  cells  interspersed.  Contained  within  the  substance 
of  the  membrane  are  numerous  microscopic  tubular  glands — the  glands 
fo  Lieberkiihn — whoso  mouths  open  on  the  free  surface.  The  solitary/ 
glands  are  small  spherical  bodies  about  the  size  of  a  mustard  seed. 


DISSECTION    OF    THE   ABDOMEN, 


309 


They  are  covered  by  the  epithelium,  and  occur  throughout  the  whole 

intestine,  but  are  more  numerous  in  the  ileum  than  in  the  jejunum. 

They  are  composed  of  lymphoid  tissue.      The  glands  of  Peyer,  or,  as 

they  are  commonly  called,  Peyer\  patches,  are  circular  or  oval  patches 

formed  by  the  aggregation  of  solitary  glands. 

They  are  more  numerous  in  the  ileum  than 

in  the  jejunum,  their  total  number   being 

about  one  hundred.     They  are  distributed 

along   the    convex    or   free   border    of   the 

intestine,    and   hence  it  was   directed   that 

the   bowel   should    be   opened    along    the 

attachment  of  the  mesentery,  so  as  to  leave 

the  patches  intact. 

Directions. — The  caecum,  with  the  first  few 

inches  of  the  double  colon  and  a  like  length 
at  the  end  of  the  small  intestine,  should  be 
separated  from  the  rest  of  the  intestinal 
mass.  After  the  serous  and  muscular  coats 
have  been  observed  on  the  inflated  csecumj 
the  bowel  should  be  slit  open  on  the  convex 
side  of  its  crook,  the  incision  being  extended 
to  its  point.  The  mucous  surfiice  is  to  be 
gently  washed;  and  in  connection  with  its 
study,  the  student  is  to  examine  the  two 
orifices  found  on  the  concave  side  of  the 
crook. 

Structure  of  the  Large  Intestine. 
Throughout  nearly  the  whole  of  its  length, 
the  wall  of  the  large  bowel  is  made  up  of 
four  coats,  similar  to  those  of  the  small 
intestine. 

1.  The  Serous  Coat  is  derived  from  the 
peritoneum,  but  it  forms  here  a  less  com- 
plete investment  than   in   the  case  of  the 

small  intestine,  considerable  areas  of  the  wall  being  without  this  cover- 
ing. Thus,  it  is  absent  where  the  caecum  and  double  colon  adhere  to 
the  pancreas  and  abdominal  parietes  in  the  sublumbar  region;  it  is  also 
absent  where  these  two  intestines  adhere  to  each  other,  and  where  the 
parallel  portions  of  the  double  colon  come  into  contact;  and  lastlv,  as 
will  be  seen  in  the  dissection  of  the  pelvis,  the  terminal  part  of  the 
rectum  is  without  a  peritoneal  covering. 

2.  The  Muscular  Coat  consists  of  two  distinct  layers — an  external  longi- 
tudinal and  an  internal  circular.  Throughout  nearly  the  whole  extent 
of  the  large  intestine,  the  longitudinal  fibres  are  not  uniformly  distri- 


'^M 


Fig.  36. 

Vertical  Section  through  the 
Wall  of  the  Duodenum,  show- 
ing THE  Glands  of  Brunner 
{Tamer). 

v.  Intestinal  villi  ;  L.  Layer  of 
glands  of  Lieberkuhn  ;  B.  A  Brun- 
ner's  gland,  d.  its  excretory  duct; 
S.M.  Submucous  coat ;  M.  Muscular 
coat. 


310 


THE    ANATOMY    OF    THE    HORSE. 


buted  over  the  wall,  but  are  collected  into  distinct  bands,  the  areas 
between  the  bands  being  provided  only  with  circular  fibres.  When  these 
bands  contract,  they  shorten  the  intestine,  and  throw  the  wall  of  the 
bowel  between  them  into  alternate  ridges  and  fuiTows.  The  number  of 
these  bands  is  difierent  at  different  points.  The  caecum  has  four.  The 
colon  in  its  1st  part  has  also  four.  Three  of  these  disappear  on  the 
2nd  part,  so  that  at  the  pelvic  flexure  there  is  only  a  single  band,  on 
the  concave  side  of  the  flexure.  This  single  band  is  continued  along 
the  3rd  part,  and  near  the  diaphragmatic  flexure  other  two  bands 
originate.  The  4th  part  has  three  bands.  The  single  colon  has  two 
bands,  one  on  each  curvature,  and  these  are  continued  on  the  first  half 
of  the  rectum,  but  are  lost  on  its  terminal  half,  as  will  be  seen  in  the 
dissection  of  the  pelvis.  The  inner  layer  of  circular  fibres  is  iniiformly 
distributed. 

3.  The  Submiicous  Coat  is  a  layer  of  loose  areolar  tissue  uniting  the 
muscular  and  mucous  coats. 

4.  The  Mucous  Coat  lines  the  cavity  of  the  bowel.  Its  surface  is 
covered  by  a  single  layer  of  columnar  epithelium,  and  in  its  deeper  part 
it  contains^  solitary  glands  and  glands  of  Lieherkiihn,  similar  to  those  of 


Fig.  37. 

Diagrammatic  View  (magnified)  of  a  small  Portion  of  the  Mucor.s  :Me.\ 
Colon  (Allen  Thomson). 


sane  of  the 


A  small  iiortion  of  the  mucous  membrane  cut  perpendicularly  at  the  edges  is  shown  in  perspective ; 
on  the  surface  are  seen  the  orifices  of  the  crypts  of  Lieberkiihn  or  tubular  glands,  the  most  of  them 
lined  by  their  columnar  ei)ithelium,  a  few  divested  of  it  and  thus  appearing  larger  ;  along  the  sides 
the  tubular  glands  are  seen  more  or  less  equally  divided  by  the  section  ;  these  are  resting  on  a  wider 
portion  of  the  submucous  tissue,  from  which  the  blood-vessels  are  represented  as  passing  into  the 
Bpaces  between  the  glands. 


the  small  intestine.  No  Peyer's  patches  are  found  in  it;  and  it  is  with- 
out villi.  The  foldings  of  the  wall  of  the  bowel  produced  by  the  longi- 
tudinal muscular  bands  involve  all  the  coats,  and  the  interior  therefore 
shows  the  alteniately  ridged  and  furrowed  appearance  already  seen  on 
the  exterior. 


DISSECTION    OF    THE    ABDOMEN.  311 

Orifices  of  the  Crook. — These  are  the  apertures  of  communication  with 
the  ileum,  and  with  the  large  colon.  At  its  termination  the  ileum  pro- 
jects slightly  into  the  interior  of  the  csecum,  and  beneath  the  mucous 
membrane  surroimding  the  orifice,  there  is  developed  a  ring  of  mus- 
cular fibres.  This  fold  of  mucous  membrane  with  its  included  muscular 
fibres  constitutes  the  ileo-coecal  valve.  The  opening  of  communication 
with  the  colon  is  considerably  larger  than  the  preceding,  above  which 
it  is  placed. 

Directions. — The  student  must  now  retin-n  to  the  parts  left  within  the 
abdominal  cavity,  where,  without  further  dissection,  he  will  be  able  to 
examine  the  stomach,  spleen,  pancreas,  and  liver.  Should  the  stomach 
be  nearly  empty,  the  ligature  should  be  untied  from  the  cut  end  of  the 
duodenum,  and  by  means  of  bellows  that  intestine  and  the  stomach 
should  be  moderately  inflated.  To  permit  this,  it  will  not  be  necessary 
to  ligature  the  oesophagus.  At  the  present  stage  the  above-mentioned 
organs  may  be  studied  as  regards  their  form,  situation,  and  relations, 
their  structure  being  postponed  for  future  consideration. 

The  Stomach  (Plates  43  and  44)  is  the  most  dilated  segment  of  the 
alimentary  tube.  When  moderately  distended,  it  will  be  seen  to  have 
the  folloAving  configuration.  It  possesses  an  anterior  and  a  posterior 
surface,  both  being  smoothly  rounded.  It  has  a  concave  or  lesser  cur- 
vature, which  is  turned  upwards  and  to  the  right;  and  a  convex  or 
greater  curvature,  which  is  directed  downwards  and  to  the  left.  The 
left  extremity  of  the  organ  is  much  the  larger,  and  is  termed  the 
cardiac  extremity,  or  the  fundus.  The  smaller  right  end  is  termed 
the  pylo^'us.  The  stomach  occupies  the  epigastric  and  left  hj-po- 
chondriac  regions,  and  it  will  be  observed  to  have  the  following 
connections.  The  anterior  surface  is  related  to  both  the  liver  and 
the  diaphragm,  and  in  the  natural  position  looks  upwards  as  well  as 
forwards.  The  posterior  face  looks  downwards  as  well  as  backwards, 
and  before  the  removal  of  the  intestines  was  related  to  these,  and 
chiefly  to  the  gastro-hepatic  flexure  of  the  double  colon.  The  smaller 
curvature  is  fixed  to  the  liver  by  means  of  the  gastro-hepatic  omentum. 
If,  in  the  present  inverted  position  of  the  animal,  the  greater  curvature 
be  pulled  backwards,  so  as  to  separate  the  anterior  surface  from  the 
liver  and  diaphragm,  the  esophagus  will  be  found  entering  the  stomach 
at  its  lesser  curvature,  about  midway  between  the  central  point  of 
that  curvature  and  the  extremity  of  the  fundus.  The  greater  curvature 
is  related  in  its  left  half  to  the  spleen,  and  throughout  the  rest  of 
its  extent  to  the  intestines,  particularly  to  the  suprasternal  flexure  of 
the  colon,  now  removed.  The  right  extremity,  or  pylorus,  is  directly 
continued  into  the  duodenum,  a  slight  constriction  being  the  only 
outward  mark  of  their  separation.  The  left  or  cardiac  extremity 
extends  to  the  left  beyond  the  insertion  of  the  oesophagus,  and  is  related 


•512  THE    ANATOMY    OF    THE    HORSE. 

to  the  pancreas  and  base  of  the  spleen.  The  stomach  is  retained  in 
position  by  continuity  with  the  oesophagus  and  duodenum,  and  by 
certain  folds  of  peritoneum,  viz.,  the  gastro-phrenic  ligament,  and  the 
gastro-hepatic,  gastro-splenic,  and  gastro-colic  omenta.  The  gastro- 
phrenic ligament  extends  from  the  diaphragm  to  the  stomach,  around 
the  oesophageal  insertion.  The  gastro-hepatic  omentum  passes  between 
the  lesser  curvature  and  the  posterior  fissux-e  of  the  liver.  The  gastro- 
splenic  07nentum  passes  from  the  cardiac  extremity  to  the  spleen.  The 
gastro-colic  or  great  omentum  is  continuous  with  the  preceding,  and 
passes  in  the  form  of  a  loose  fold  from  the  greater  curvature.  It 
extends  backwards  and  downwards,  and  then  curves  upwai'ds  to  the 
roof  of  the  abdominal  cavity;  and,  as  has  already  been  explained 
(page  300),  it  separates  the  gi-eater  arid  lesser  cavities  of  the  perito- 
neum; and,  inasmuch  as  in  man  it  hangs  downwards  to  float  upon 
the  intestines,  it  has  been  termed  the  Epiploon.  Even  in  emaciated 
subjects,  it  contains  between  its  layers  a  considerable  quaxitity  of 
fat. 

The  Duodenum  (Plates  43  and  44).  A  better  opportunity  to  examine 
this  pai-t  of  the  ixitestine  is  now  affox-ded.  Comxncncing  in  the  epigas- 
trixxm,  at  the  pylox-ic  orifice  of  the  stomach,  it  ascends  aci'oss  the  posterior 
face  of  the  right  lobe  of  the  livex-,  in  passixig  ixxto  the  right  hypochou- 
drium.  It  then  curves  backwax'ds  in  the  x-ight  lumbar  x-egion,  beneath 
the  right  kidxiey;  and  sweeping  x-ound  the  crook  of  the  cajcum,  it  cx-osses 
the  spixie  and  is  continued  as  the  jejunum.  Its  calibre  is  gx-eatest  just 
beyoxid  the  pylorus,  and  at  tliis  point  it  presents,  when  ixiflated,  a  small 
dilatation  like  a  xnixiiatux-e  stoxuach  with  its  greater  curvatxxx-e  sxxperior. 
Thx'oughout  the  whole  of  its  course  it  is  retained  in  position  by  a  xxaxTOW 
band  of  peritoxieum  formed  by  the  seroixs  xnembrane  as  it  passes  to 
envelop  the  bowel.  The  right  extrexiiity  of  the  pancreas  rests  against  the 
duodenum,  a  few  inches  fx-om  the  pylorus,  and  at  that  point  the  wall 
of  the  bowel  is  perforated  by  the  bile  and  pancreatic  ducts. 

The  Spleen  (Plates  43  axid  44)  is  a  bluish-purple  solid  organ  placed 
ixi  close  proxixuity  to  the  left  sac  of  the  stomach.  Ixi  the  hox*se  it  has 
a  scythe-shaped  outlixie.  It  presents  an  extex'xial  face,  which  is  slightly 
coxivex ;  axi  ixiternal  face,  which  is  slightly  concave  axxd  xiarrower  thaxi 
the  oixter;  an  axxterior  thick  border;  axxd  a  postei'ior  border,  which  is 
shax'p.  Its  surfaces  are  widest  above,  whei-e  they  terminate  in  the  base 
of  the  organ,  and  below  they  taper  to  the  apex.  The  spleen  is  situated 
in  the  left  hypochoxxdx-iac  region,  and  has  the  followixxg  relatioxxs : — Its 
outei'  surface  is  x-elated  to  the  diaphragm ;  its  iixner  surface  contacts  with 
the  double  coloxx ;  its  axxterior  box'der  is  pexxetrated  by  the  vessels  and 
nerves  of  the  organ,  and  is  x-elated  to  the  greater  curvature  of  the 
stomach ;  its  posterior  border  is  free,  and  is  included  between  the  intes- 
tines and  the  diaphragm  ;  its  base  is  related  to  the  pancreas  and  left 


DISSECTION    OF    THE    ABDOMEN.  313 

kidney.  The  spleen  is  retained  in  the  left  hypochondrium  by  the  gastro- 
splenic  omentum,  and  by  a  special  sjilenic  ligament.  The  gastro-splenic 
omentum  forms  a  loose  connection  between  the  left  half  of  the  greater 
curvature  of  the  stomach  and  the  anterior  border  of  the  spleen.  The 
splenic  ligament  is  a  fold  of  peritoneum  developed  at  the  base  of  the 
organ,  and  formed  by  the  serous  membrane  in  passing  from  around  the 
anterior  end  of  the  left  kidney  to  envelop  the  spleen. 

The  Pancreas  (Plate  44)  is  a  body  having  a  lobulated  structure  and  a 
very  irregular  shape.  It  is  placed  across  the  roof  of  the  abdominal  cavity, 
its  central  portion  underlying  the  last  dorsal  vertebrae.  Its  upper  face 
is  applied  to  the  aorta,  the  coeliac  axis,  the  vena  cava,  the  pillars  of  the 
diaphragm,  and  the  right  kidney,  and  is  partly  covered  by  peritoneum. 
Its  lower  face  towards  the  right  is  adherent  to  the  crook  of  the  caecum 
and  the  termination  of  the  double  colon,  while  to  the  left  it  is  covered 
by  peritoneum.  Its  anterior  border  is  related  to  the  stomach,  the 
duodenum,  and  the  liver.  Its  posterior  border  is  related  about  its  centre 
to  the  anterior  mesenteric  artery.  Its  right  extremity,  or  head,  is  in 
contact  with  the  duodenum;  while  the  left  extremity,  or  tail,  is  related 
to  the  base  of  the  spleen.  The  entire  thickness  of  the  gland  is  perforated 
by  the  portal  vein,  which  passes  from  its  lower  to  its  upper  surface 
through  what  is  named  the  pancreatic  ring.  The  gland  possesses  two 
excretory  ducts,  both  of  which  leave  it  at  its  right  extremity.  The  main 
duct  is  named  the  duct  of  Wirsimg,  and  it  perforates  the  wall  of  the 
duodenum  about  six  inches  from  the  pylorus,  and  close  by  the  point  of 
entrance  of  the  bile  duct.  The  accessory  duct  is  much  smaller,  and  pene- 
trates the  bowel  at  a  point  opposite  the  entrance  of  the  duct  of  Wirsung. 
The  healthy  fresh  pancreas  has  a  greyish-yellow  colour ;  but  when 
decomposition  sets  in  after  death,  this  speedily  changes  to  an  almost 
black  hue. 

The  Liver  (Plates  43  and  44)  is  the  largest  gland  in  the  body.  It 
forms  the  bile  and  discharges  it  into  the  duodenum.  In  health  it  has  a 
reddish-brown  colour  and  a  moderately  firm  consistence.  In  form  it  is 
not  comparable  to  any  common  object,  and  its  irregularity  of  shape  makes 
its  description  somewhat  difficult.  It  should  be  observed,  in  the  first 
place,  that  inferiorly  the  rim  of  the  organ  is  deeply  indented,  or 
notched,  and  two  of  the  largest  of  these  notches  serve  to  partially 
divide  the  gland  into  its  three  main  lobes,  viz.,  a  right,  a  left,  and 
a  middle,  or  lobulus  quadratus.  Of  these  the  middle  lobe  is  always 
the  smallest,  and  its  inferior  border  shows  two  or  three  minor  indenta- 
tions. The  left  lobe  is  generally  the  largest,  but  sometimes  it  is  less 
than  the  right.  The  liver  possesses  a  fourth  lobe,  in  the  form  of 
a  small  projection  of  liver  substance  about  the  size  of  two  or  three 
of  the  human  fingers,  and  situated  at  the  vipper  part  of  the  right  lobe. 
This  is  the  homologue  of  the  lobulus  caudatus  of  the  human  subject. 


314  THE  ANATOMY  OF  THE  HORSE. 

The  exact  form  of  the  liver  will  be  more  distinctly  seen  when  it  has 
been  removed  from  the  body ;  but  while  it  remains  in  situ,  the  student 
may  endeavour  to  make  out  the  following  points : — Viewing  the  organ 
as  a  whole,  it  may  be  described  as  having  an  anterior  and  a  posterior 
siirfacc,  and  a  circumference  divisible  into  an  upper  and  a  lower  border. 
The  anterior  surface  is  closely  applied  to  the  diaphragm,  and  is  convex. 
The  posterior  vena  cava,  in  descending  from  the  spine  to  the  foramen 
dextrum,  passes  between  this  surface  and  the  diaphragm ;  and  its 
course  is  marked  on  the  liver  by  a  vertical  groove,  which  may  be 
termed  the  anterior  fissure.  The  posterior  suiface,  when  the  organ 
is  in  situ,  is  concave ;  but  when  the  liver  is  removed  from  the  body, 
this  surface,  like  the  anterior,  is  slightly  convex.  It  presents  the  ]jortal 
fissure  (L.  porta,  a  gate),  by  which  the  portal  vein,  hepatic  artery,  bile 
duct,  and  hepatic  nerves  and  lymphatics  enter  the  liver.  The  upper 
border  shows  about  its  centre  a  roxmded  notch  for  the  reception  of 
the  short  abdominal  portion  of  the  oesophagus.  The  lower  border  shows 
the  sharper  and  deeper  indentations  dividing  the  liver  into  its  three 
principal  lobes,  and  the  lesser  indentations  that  partially  subdivide  the 
lobulus  quadratus. 

The  liver  is  situated  in  the  epigastric  and  right  and  left  hypo- 
chondriac regions.  Its  most  important  relations,  besides  those 
already  mentioned,  are  as  follows  : — The  anterior  surface  is  applied 
to  the  diaphragm,  the  right  lobe,  which  has  the  highest  point 
of  contact,  being  related  to  the  most  superior  part  of  the  muscular 
rim  on  the  right  side,  the  lobulus  quadratus  corresponding  to  the 
phrenic  centre,  while  the  left  lobe  touches  the  lowest  point  reached 
by  the  liver,  and  lies  against  the  lower  part  of  the  muscular  rim 
on  the  left  side  and  the  adjacent  part  of  the  tendinous  centre. 
The  posterior  surface  is  related  to  the  stomach,  the  duodenum,  the 
gastro-hepatic  flexure  of  the  double  colon,  the  pancreas,  and  the 
right  kidney,  the  latter  slightly  indenting  the  upper  part  of  the  right 
lobe. 

If  an  attempt  be  made  to  pull  the  liver  from  its  position,  it  will 
be  found  that  this  is  opposed  by  certain  folds  of  peritoneum  which 
pass  between  it  and  the  abdominal  parietes.  These  are  the  ligaments 
of  the  liver,  and  they  are  named  as  follows: — 

The  Biffht  Lateral  Ligament  passes  between  the  right  lobe  and  the 
adjacent  part  of  the  phrenic  rim. 

The  Left  Lateral  Ligament  attaches  the  left  lobe  to  the  phrenic 
centre. 

The  Falciform  or  Suspensory  Lir/ament  attaches  the  lobulus  quadratus 
to  the  diaphragm  and  to  the  abdominal  floor  a  little  to  the  right  of  the 
linea  alba.  Its  posterior  edge  is  concave  and  free,  and  contains  the 
shrivelled  remains  of  the  umbilical  vein — the  so-called  round  ligament. 


DISSECTION    OF    THE    ABDOMEN.  315 

The  Ligament  of  the  Caudate  Lobe  is  a  small  peritoneal  fold  passing 
between  the  anterior  end  of  the  right  kidney  and  the  lobulus  caudatus. 

The  Coronary  Ligament. — If  all  the  preceding  ligaments  be  cut,  and 
an  attempt  made  to  pull  the  liver  out  of  position,  it  will  be  found  that 
the  ghand  is  still  finally  attached  to  the  diaphragm  by  its  anterior  face. 
This  adhesion  takes  place  over  an  area  that  is  traversed  by  the  anterior 
fissure  lodging  the  vena  cava,  and  the  peritoneum  in  passing  between 
the  gland  and  the  phrenic  centre  on  each  side  of  this  area  constitutes 
the  coronary  ligament. 

Directions. — The  coeliac  trunk  and  its  branches  must  now  be  prepared ; 
and,  coincidently  with  this,  the  bile  duct,  portal  vein,  and  solar  plexus 
must  be  dissected.  The  portal  vein  will  be  found  passing  through 
the  pancreas  to  the  transverse  fissure;  and  emerging  from  the  fissure, 
below  the  vein,  is  the  bile  duct,  which  passes  to  open  into  the  duodenum 
close  by  the  principal  pancreatic  duct.  The  coeliac  axis  is  concealed 
by  the  pancreas,  which  must  be  carefully  raised  by  dissection  at  its 
anterior  border,  and  pulled  backwards.  The  same  dissection  will 
expose  the  semilunar  ganglia  and  the  solar  plexus,  whose  branches  are 
to  be  traced  in  company  Avith  the  arteries.  In  dissecting  the  vessels, 
the  student  will  meet  the  lymphatic  glands  of  the  stomach,  spleen,  and 
liver. 

Lymphatics.  The  glands  of  the  stomach  form  two  groups,  viz.,  (1) 
a  few  large  glands  situated  at  the  lesser  curvature,  and  (2)  a  number  of 
smaller  glands  placed  at  the  greater  cm-vature.  The  glands  of  the 
liver  also  form  two  groups,  viz.,  (1)  a  nixmber  situated  in  the  posterior 
fissure,  and  (2)  a  group,  between  the  portal  vein  and  the  pancreas. 
The  glands  of  the  spleen  are  placed  on  the  course  of  the  splenic  vessels. 
The  lymphatic  vessels  emanating  from  the  stomach,  liver,  and  spleen 
traverse  these  groups  of  glands;  and  after  anastomosing  with  each 
other,  they  pass  to  the  thoracic  duct. 

The  Bile  Duct.  This  is  the  main  duct  for  the  conveyance  of  the 
bile  from  the  liver  to  the  intestine.  It  is  formed  at  the  portal  fissure 
of  the  liver,  by  the  union  of  secondary  branches  from  the  three  principal 
lobes,  and  it  passes  between  the  layers  of  the  gastro-hepatic  omentum  to 
penetrate  the  wall  of  the  duodenum,  about  six  inches  from  the  pylorus. 
The  excretory  apparatus  of  the  horse's  liver  has  the  peculiarity — shared 
by  a  few  other  animals — of  being  without  a  gall-bladder. 

The  Cceliac  Axis  (Plates  43  and  44)  is  a  collateral  branch  of  the 
abdominal  aorta,  arising  from  the  inferior  face  of  that  vessel  between 
the  pillars  of  the  diaphragm.  It  is  less  than  an  inch  in  length,  and  it 
divides  into  three  branches :  the  gastric  trunk,  the  hepatic  artery,  and 
the  splenic  artery. 

The  Hepatic  Artery  is  directed  obliquely  forwards,  downwards,  and 
to  the  right,  to  gain  the  posterior  fissure  of  the  liver,  which  it  penetrates 


316  THE   ANATOMY    OF    THE    HORSE. 

in  company  with  the  portal  vein  and  the  bile  duct.  At  first  imbedded 
in  the  pancreas,  it  then  passes  over  the  duodenum,  and  reaches  its  des- 
tination by  passing  between  the  layers  of  the  gastro-hepatic  omentum. 
It  crosses  the  posterior  vena  cava,  from  which  it  is  separated  by  the 
foramen  of  Winslow.     It 'gives  off  the  following  collateral  branches  : — 

1.  Pancreatic  Branches. 

2.  The  Ri[iht  (jastro-omental  Artery,  which  is,  at  its  origin,  of  larger 
volume  than  the  continuation  of  the  parent  trunk,  crosses  behind 
the  duodenum ;  and  placing  itself  in  the  texture  of  the  great  omentum, 
it  is  carried  round  the  greater  curvature  of  the  stomach  to  inosculate  with 
the  left  gastro-omental  artery.  It  gives  off  the  pyloric  and  duodenal 
arteries,  besides  numerous  omental  and  gastric  branches.  The  pyloric 
artery  is  detached  from  the  right  gastro-omontal  artery  near  its  origin, 
and  sometimes  it  is  a  branch  of  the  hepatic  artery.  It  supplies  the 
pylorus  and  the  initial  dilatation  of  the  duodenum.  The  duodenal 
artery  is  detached  from  the  right  gastro-omental  artery  before  that  vessel 
crosses  the  duodenum ;  and  following  the  lesser  curvature  of  the 
duodenum,  in  the  narrow  serous  band  that  fixes  the  bowel,  it  meets, 
and  inosculates  with,  the  first  artery  from  the  left  branch  of  the  anterior 
mesenteric  artery.  The  omental  branches  of  the  right  gastro-omental 
are  small  and  unimportant.  The  gastric  branches  pass  from  the 
concave  side  of  the  parent  artery ;  and  bifurcating  at  the  greater 
curvature,  they  are  distributed  to  the  right  sac  of  the  stomach  on  both 
its  surfaces,  where  they  anastomose  with  branches  of  the  pyloric  and 
gastric  arteries. 

The  Gastric  Trunk  is  the  central  of  the  three  terminal  branches  of 
the  coeliac  axis.  After  a  course  of  a  few  inches  downwards  and  forwards, 
it  bifurcates  to  form  the  anterior  and  posterior  gastric  arteries. 

The  Anterior  Gastric  Artery  reaches  the  antei'ior  surface  of  the 
stomach  by  crossing  the  lesser  curvature  immediately  to  the  right  of 
the  oesophagus. 

The  Posterior  Gastric  Artery  descends  to  the  lesser  curvature  of  the 
stomach,  where  it  divides  into  branches  distributed  on  the  posterior 
aspect  of  the  organ. 

The  Pleuro-oesopluirjeal  Artery  is  a  vessel  constantly  present,  but 
variable  as  regards  its  origin.  It  may  arise  from  the  gastric  trunk  or 
one  of  its  branches,  or  from  the  splenic  artery.  Passing  through  the 
foramen  sinistrum  along  with  the  oesophagus,  it  enters  the  thoracic 
cavity,  and  there  anastomoses  with  the  oesophageal  arteries,  supplying 
the  pulmonary  pleui'a  at  the  base  of  the  liuig. 

The  Splenic  Artery  is  considerably  larger  than  the  gastric  trunk  or 
the  hepatic  artery.  Under  cover  of  the  pancreas,  it  passes  outwards 
between  the  left  kidney  and  the  cardiac  extremity  of  the  stomach. 
Reaching   the   spleen,  it  descends  along  the  anterior  border   of   that 


DISSECTION    OF    THE    ABDOMEN.  317 

organ,  beyond  which  it  is  continued  as  the  left  gastro-omental  artery, 
From  its  convex  side  it  gives  off  many  large  splenic  branches  ;  and  from  its 
concave  side  it  emits  gastric  branches,  which  pass  in  the  gastro-splenic 
omentum  to  reach  the  great  curvature  of  the  stomach,  where  they 
bifurcate  to  be  distributed  to  both  surfaces  of  the  left  sac. 

The  Left  Gastro-omental  Artery  is  the  continuation  of  the  splenic 
artery  beyond  the  tip  of  the  spleen.  It  passes  in  the  texture  of  the 
great  omentum  to  meet,  and  inosculate  with,  the  right  gastro-omental 
artery,  advancing  in  the  opposite  direction.  Besides  omental  branches,  it 
emits  gastric  branches,  which  bifurcate  at  the  great  curvature  of  the 
stomach  to  be  distributed  to  both  its  surfaces. 

The  Portal  Vein  (Plates  43  and  44)  is  the  trunk  which  collects  the 
blood  from  the  stomach,  intestines,  spleen,  and  pancreas,  and  conveys 
it  to  the  liver,  where,  as  will  afterwards  be  described  in  connection  with 
the  liver  structure,  the  vessel  comports  itself  after  the  manner  of  an 
artery.  The  vessel  is  formed  behind  the  pancreas,  by  the  junction  of 
the  anterior  mesenteric  vein  with  a  short  trunk  resulting  from  the  union 
of  the  posterior  mesenteric  and  sj^lenic  veins.  It  gains  the  upper  face  of 
the  pancreas  by  passing  through  its  substance,  the  perforation  being 
termed  the  pancreatic  ring;  and  descending  in  the  gastro-hepatic 
omentum  to  the  posterior  fissure  of  the  liver,  it  penetrates  the  substance 
of  the  gland  in  company  with  the  bile  duct  and  hepatic  artery. 

Anterior  and  Posterior  Mesenteric  Veins,  satellites  of  the  arteries  of  the 
same  names,  have  already  been  described ;  but  there  is  no  venous  trunk 
corresponding  to  the  coeliac  axis,  the  companion  veins  of  the  divisions  of 
that  artery  behaving  as  follows  : — 

The  Splenic  Vein  is  the  upward  continuation  of  the  left  gastro-omental 
vein.  After  receiving  the  j)osterior  gastric  vein,  it  becomes  one  of  the 
roots  of  the  vena  portse,  previously  forming  a  short  trunk  by  union  with 
the  posterior  mesenteric  vein. 

The  Anterior  Gastric  Vein  joins  the  vena  portae  in  the  posterior  fissure 
of  the  liver. 

The  Eight  Gastro-omental  Vein  is  continuous  with  the  left  vein  of  the 
same  name,  in  the  texture  of  the  great  omentum,  opposite  the  middle  of 
great  curvature  of  the  stomach.  It  receives  gasti'ic,  omental,  duodenal, 
pyloric,  and  pancreatic  branches,  all  of  which  run  in  company  with  the 
arteries  of  the  same  names;  and  then,  above  the  pancreas,  it  joins  the 
portal  vein. 

The  Qi^soPHAGEAL  Nerves.  These  nerves  are  the  backward  continu- 
ations of  the  vagus,  pneumogastric,  or  10th  cranial  nerves.  They  reach 
the  abdominal  cavity  by  passing  through  the  foramen  sinistrum  of  the 
diaphragm,  in  company  with  the  cESophagus  and  the  pleuro-oesophageal 
branch  of  the  gastric  artery.  The  inferior  nerve  forms  at  the  lesser 
cui-vature  of  the  stomach  a  plexus  whose  filaments  pass  mainly  to  the 


318  THE    AX  ATOMY    OF    THE    HORSE. 

right  sac;  while  the  superior,  after  giving  branches  to  the  left  sac, 
joins  the  solar  plexus. 

The  Splanchnic  Nerves.  On  each  side  there  are  two  splanchnic 
nerves — a  great  and  a  small  Both  are  formed  by  efferent  branches  of 
the  dorsal  portion  of  the  sympathetic  gangliated  cord,  and  they  reach 
the  abdomen  by  passing  between  the  diaphragm  and  the  psoas  parvus 
muscle.  The  great  splanchnic  nerves  terminate  in  the  semilunar 
ganglia;  the  small  nerves  pass  directly  to  the  solar  plexus,  or  they  may 
be  continued  to  the  renal  or  the  suprarenal  plexus. 

The  Semilunar  Ganglia  are  the  largest  in  the  body.  They  are 
placed  one  at  each  side  of  the  lower  face  of  the  aorta,  between  the 
coeliac  and  anterior  mesenteric  arteries.  Each  receives  the  great 
splanchnic  nerve  of  its  own  side,  and  the  two  ganglia  communicate  by 
transverse  branches  across  the  lower  face  of  the  aorta.  The  efferent 
branches  which  proceed  from  them  form  the  solar  plexus. 

The  Solar  Plexus  is  an  intricate  network  of  nerves  and  ganglia.  It  is 
joined  on  each  side  by  the  lesser  splanchnic  nerve,  and  by  the  terminal  fila- 
ments of  the  superior  oesophageal  nerve.  From  the  plexus  nerves  pass  to 
the  abominal  viscera,  and  in  doing  so  they  run  in  company  with  arteries. 
There  is  thus  :  a  coeliac  plexus,  whose  branches  i-each  the  liver,  pancreas, 
spleen,  and  stomach,  by  accompanying  the  divisions  of  the  hepatic, 
splenic,  and  gastric  arteries ;  a  renal  and  a  suprarenal  plexus,  which  pass 
to  the  kidneys  and  suprarenal  bodies  ;  an  aortic  j^l^xus,  continued  back- 
wards on  the  aorta  to  join  the  posterior  mesenteric  plexus;  and  an 
anterior  mesenteric  plexus,  already  described. 

Directions. — The  form,  situation,  and  relations  of  the  kidneys,  and  the 
coiirse  of  the  ureters  should  now  be  examined.  AVithout  displacing  the 
kidneys,  the  fat  and  })eritoneum  is  to  be  stripped  from  their  lower  face, 
their  vessels  being  carefully  cleaned  at  the  same  time.  The  ureter  will 
be  found  ])assing  backwards  from  the  notched  inner  border  of  each 
kidne}',  and  it  is  to  be  followed  backwards  to  the  entrance  to  the  pelvis. 
In  close  relation  to  each  kidney  is  its  suprarenal  body,  which,  to  prevent 
displacement,  may  be  transfixed  in  position  with  a  long  pin. 

The  Kidneys  (Plates  44  and  47)  are  the  two  glandular  bodies  that 
secrete  the  urine.  Each  kidney  occupies  a  position  at  the  side  of  the 
vertebral  column,  on  the  inferior  aspect  of  the  loins,  and  at  the  roof  of  the 
abdominal  cavity.  In  all  except  the  most  emaciated  subjects,  the  kidneys 
are  surrounded  by  a  (piantity  of  adipose  tissue,  which  is  so  abundant  in 
fat  animals  as  to  completely  isolate  them  from  surrounding  objects.  The 
most  common  shape  of  the  mammalian  kidney  is  so  well  known  that  it  is 
popularly  used  as  a  descriptive  term,  objects  having  a  similar  form  being 
described  as  "kidney-shaped."  Each  kidney  possesses  two  surfaces,  two 
borders,  and  two  extremities.  The  inferior  surface  is  convex ;  the 
superior,  which  is  concealed  at  present,  is  almost  flat.     The  outer  border 


DISSECTION    OF    THE    ABDOMEN.  319 

is  convex ;  while  the  inner  is  concave,  presenting  a  well-marked  notch 
termed  the  Jiilus.  From  this  hilus  the  ureter  issues,  and  in  its  neigh- 
boui'hood  the  renal  vessels  and  nerves  pass  into  or  out  of  the  kidney. 
The  extremities  are  anterior  and  posterior,  and  both  are  rounded.  It 
will  at  once  be  noticed,  however  (Plate  47),  that  although  this  description 
applies  to  both  kidneys,  they  are  far  from  being  identical  in  shape.  The 
right  kidney  has  an  outline  somewhat  like  the  "  heart "  of  playing-cards, 
while  the  left  has  a  decided  resemblance  to  a  haricot  bean.  The  right 
has  the  longest  ti-ansverse,  but  the  shortest  antero-posterior,  diameter. 
The  right  is  nearly  symmetrical  on  each  side  of  a  line  drawn  from  the 
hilus  to  the  middle  of  the  outer  border;  but  if  such  a  line  be  di-awn  on 
the  left,  the  part  in  front  of  the  line  will  be  considerably  smaller  than 
the  part  behind  it. 

Furthermore,  it  will  be  noticed  that  the  two  kidneys  differ  in  situation, 
and  in  relations.  The  right  kidney  is  the  more  anterior;  and  taking 
their  relation  to  the  skeleton,  the  difference  may  be  expressed  thus :  the 
right  extends  from  the  middle  of  the  third  last  intercostal  space  at  its 
upper  end  to  a  point  beneath  the  2nd  lumbar  transverse  process;  the 
left  extends  from  the  second  last  intercostal  space  to  the  3rd  lumbar 
transverse  process. 

The  right  kidney  is  related  by  its  upper  face  to  the  psoas  muscles  and 
to  the  rim  of  the  diaphragm ;  by  its  lower  face  it  contacts  with  the 
pancreas  and  the  crook  of  the  ctecum,  and  is  partly  covered  by  peri- 
toneum ;  its  inner  border  is  margined  by  the  posterior  vena  cava,  and  is  in 
contact  with  the  right  suprarenal  capsule  in  front  of  the  hilus  (some- 
times behind);  its  outer  border  is  in  contact  with  the  dxiodenum;  its 
anterior  extremity  is  in  contact  with  the  right  lobe  of  the  liver  (which  is 
slightly  depressed  for  it)  and  with  the  lobulus  caudatus,  and  to  the  latter 
a  small  fold  of  peritoneum  passes  from  the  lower  face  of  the  kidney. 

The  left  kidney  has  the  same  relations  on  its  upper  face  as  the  right ; 
its  lower  face  is  covered  by  peritoneum,  and  is  related  to  the  small 
intestines ;  its  inner  border  is  margined  by  the  aorta,  and  is  related  to 
the  left  suprarenal  body  in  front  of  the  hilus;  its  anterior  border  is 
related  to  the  left  extremity  of  the  pancreas ;  and  the  anterior  half  of 
its  outer  or  convex  border  is  related  to  the  base  of  the  spleen. 

Renal  Arteries.  Each  kidney  receives  blood  from  a  large  vessel — 
the  renal  artery — which  is  a  branch  of  the  aorta.  Springing  at  a  right 
angle  from  the  parent  trunk,  the  artery  passes  towards  the  hilus,  where 
it  divides  into  a  number  of  branches  that  penetrate  the  kidney  substance. 
The  left  artery  is  short,  and  passes  directly  to  the  hilus ;  the  right  is 
longer,  and  passes  between  the  psoas  parvus  muscle  and  the  vena  cava 
to  reach  its  destination.  It  is  also  generally  a  little  anterior  to  the  left 
in  its  point  of  detachment,  both  being  a  little  behind  the  trunk  of  the 
anterior  mesenteric  artery. 


320  THE  ANATOMY  OF  THE  HORSE. 

The  Renal  Veins  are  as  large  in  proportion  as  the  arteries.  They 
join  tlie  posterior  vena  cava,  the  right  vein  being  the  shorter  and 
passing  directly  from  the  hilus,  while  the  left  crosses  the  lower  face  of 
the  aorta  behind  the  anterior  mesenteric  artery. 

The  Renal  Plexus  of  nerves  interlace  around  the  artery,  and  enter 
the  kidney  with  its  branches. 

The  Suprarenal  Capsules  (Plates  44  and  47).  These  are  two 
small  solid  bodies  found  in  close  relation  to  the  kidneys,  the  right  being 
between  the  vena  cava  and  the  inner  border  of  the  right  kidney,  and  the 
left  between  the  aorta  and  left  kidney.  They  have  an  irregular  elon- 
gated shape,  and  a  slatey-brown  colour.  They  ai-e  highly  vascular, 
receiving  branches  from  the  mesenteric  or  renal  arteries,  and  having 
veins  that  enter  the  posterior  vena  cava  or  the  renal  veins.  They 
have  also  a  rich  nervous  supply,  receiving  the  suprarenal  plexus — an 
offset  from  the  solar  plexus.  Tlieir  substance  consists  of  a  cortical  and 
a  medullary  portion.  Each  possesses  a  fibrous  capsule  continuous  with 
an  internal  trabecular  framework.  The  interspaces  of  these  trabecule 
contain  nucleated  polyhedral  or  branched  cells,  which  in  the  medulla 
and  innermost  layer  of  the  cortex  frequently  enclose  yellowish-brown 
pigment.     They  are  without  ducts,  and  their  fuAction  is  not  well  known. 

The  Ureters.  The  ureter  is  the  tube  which  conveys  the  urine  from 
the  pelvis  of  the  kidney  to  the  urinary  bladder.  On  the  left  side  it  has 
the  following  course.  Beginning  at  the  hilus,  it  is  directed  backwards 
and  inwards  across  the  lower  face  of  the  kidney  to  place  itself  at  the 
side  of  the  aorta,  over  (under,  in  the  natural  position)  the  psoas 
parvus  muscle.  Here  it  is  crossed  obliquely  by  the  spermatic  artery. 
It  next  curves  a  little  outwards,  crossing  over  the  circumflex  iliac 
artery  and  the  artery  of  the  cord,  runs  at  the  outer  side  of  the 
external  iliac  artery,  and  then  crosses  it  very  obliquely  to  enter  the 
pelvis.  The  right  ureter  has  similar  relations,  except  that  it  passes 
at  the  side  of  the  vena  cava  instead  of  the  aorta. 

Directions. — Pin  each  ureter  in  position  immediately  behind  the 
kidney,  a:id  then  cut  it  across.  Remove  carefully  the  liver,  stomach, 
duodenum,  spleen,  pancreas,  and  kidneys,  by  cutting  the  oesophagus 
and  the  various  ligaments,  vessels,  and  cellular  adhesions  which  retain 
these  organs  in  position.  Put  them  in  carbolic  solution,  or  procure 
fresh  organs,  to  serve  for  the  examination  of  their  structure.  In  the 
meantime  proceed  to  dissect  the  sublumbar  region. 

THE    SURLUMBAR    REGION. 

Directions. — Under  this  heading  there  will  be  described  the  abdomi- 
nal aorta  and  the  vena  cava,  witli  their  branches  ;  the  inferior  primary 
branch  of  the  last  dorsal  nerve,  and  the  corresponding  branches  of  the 
first  four  lumbar  nerves;  the  aortic  plexus  and  gangliated  lumbar  cord 


DISSECTION    OF   THE   ABDOMEN.  321 

of  the  sympathetic  nerve  ;  and,  lastly,  a  group  of  muscles,  comprising 
the  iliacus,  psoas  magnus,  psoas  parvus,  quadratus  lumborum,  and 
lumbar  intertransverse  muscles.  The  great  arterial  and  venous  trunks 
are  mesially  placed,  and  the  aortic  plexus  is  on  the  great  artery.  The 
other  structures  enumerated  are  the  same  on  both  sides  of  the  body. 
One  side  may  be  used  for  the  nerves  and  the  arterial  and  venous 
branches,  the  other  being  reserved  for  the  muscles.  In  the  mare,  after 
the  ovarian  and  uterine  vessels  have  been  examined,  the  broad  ligaments 
must  be  cut  to  allow  of  the  ovaries  and  uterus  being  pushed  into  the 
pelvic  cavity,  where  they  are  to  remain  until  they  can  be  dissected  along 
with  the  other  reproductive  organs  of  the  female. 

Lymphatic  Glands.  In  cleaning  the  nerves  and  vessels,  the  follow- 
ing groups  of  lymphatic  glands  will  be  found : — 1.  Sacral  glands, 
between  the  right  and  left  internal  iliac  arteries  at  the  entrance  to  the 
pelvis.  2.  Internal  Iliac  glands,  between  and  around  the  roots  of  the 
external  and  internal  iliac  arteries  on  each  side.  3.  External  Iliac 
glands  towards  the  point  of  bifurcation  of  the  circumflex  iliac  artery. 
4.  Lumbar  glands,  on  the  lower  face  of  the  aorta  around  the  roots  of  the 
posterior  mesenteric  and  spermatic  arteries.  These  various  groups  of 
glands  are  placed  on  the  course  of  the  lymphatic  vessels  of  the  hind 
limb,  pelvis,  and  spermatic  cord ;  and  the  efferent  vessels  from  the  most 
anterior  group  (lumbar)  pass  to  enter  the  receptaculum  chyli. 

The  Posterior  Aorta  (Plates  44  and  45).  The  abdominal  portion  of 
this  great  artery  appears  close  to  the  spine,  between  the  two  pillars  of  the 
diaphragm,  the  opening  being  termed  the  hiatus  aorticus.  It  passes  back- 
wards across  the  lumbar  vertebral  bodies,  resting  on  the  left  pillar  of  the 
diaphragm  and  the  inferior  common  ligament.  At  the  5th  lumbar  vertebra 
it  terminates  in  four  branches,  two  diverging  to  each  side.  These  are 
the  external  and  internal  iliac  arteries.  On  its  right  side  the  aorta  is 
related  to  the  vena  cava.  On  the  left  it  is  related  to  the  psoas  parvus, 
the  left  lumbar  sympathetic  cord,  and  the  left  kidney  and  suprarenal 
capsule  ;  and  the  left  ureter  is  beside  or  in  actual  contact  with  it.  Besides 
the  external  and  iliac  arteries,  which  are  described  as  its  terminal 
branches,  it  gives  off"  the  following  : — 

1.  Phrenic  Brcmches  (two  or  three)  to  the  pillars  of  the  diaphragm. 
They  are  given  off"  at  the  hiatus  aorticus. 

2.  Lumbar  Arteries.  There  are  six  or  seven  of  these  on  each  side. 
The  last  comes  from  the  lateral  sacral  artery,  the  second  last  from  the 
internal  iliac,  and  the  others  from  the  aorta.  These  last  arise  from 
the  upper  aspect  of  the  vessel,  and  divide  into  two  branches — a  superior 
for  the  skin  and  muscles  over  the  lumbar  vertebrae,  giving  also  a  spinal 
twig  through  the  intervertebral  foramen;  and  an  inferior  which  passes  out- 
wards in  the  intertransverse  spaces  to  the  flank,  where  it  anastomoses 
with  the  circumflex  iliac  ai'tery  in  supplying  the  abdominal  muscles. 

Y 


322  THE  ANATOMY  OF  THE  HORSE. 

3.  The  Middle  Sacral  Artery  is  an  extremely  slender  vessel,  and  not 
always  present.  Search  for  it  in  the  angle  between  the  internal  iliacs. 
Arising  from  the  summit  of  that  angle,  it  passes  mesially  backwards  on 
the  sacrum.  It  is  of  interest  as  representing  the  large  vessel  which  in 
some  animals  continues  the  aorta  to  the  coccygeal  region. 

4.  The  Coeliac  Axis  is  detached  as  soon  as  the  aorta  passes  through 
the  hiatus  aorticus. 

5.  The  Anterior  Mesenteric — the  largest  of  the  branches — is  detached 
at  the  1st  lumbar  vez-tebra. 

6.  The  Eenal  Arteries,  right  and  left,  arise  from  the  sides  of  the 
aorta  at  the  articulation  between  the  1st  and  2nd  lumbar  vertebra). 

7.  The  Spermatic  Arteries,  right  and  left,  come  off  a  few  inches 
behind  the  renals,  viz.,  between  the  3rd  and  4th  lumbar  vertebrae,  and 
one  generally  a  little  in  advance  of  the  other.  As  seen  in  the 
dissection,  each  passes  obliquely  backwards  and  outw^ards  over  the 
ureter  and  circumflex  iliac  artery  to  gain  the  internal  abdominal  ring, 
where  it  joins  the  other  constituents  of  the  spermatic  cord.  In  the 
cord  it  has  a  remarkably  tortuous  disposition ;  and,  although  a  long 
vesssel,  it  detaches  no  branches  of  any  size  until  it  reaches  the  testicle. 
In  the  mare  it  is  represented  by  the  ovarian  artery,  which  passes  in  a 
tortuous  manner  between  the  layers  of  the  uterine  broad  ligament  to 
reach  the  ovary.     It  gives  off  a  uterine  branch  to  the  uterine  horn. 

8.  The  Posterior  Mesenteric  Artery.  This  vessel  is  usually  detached 
at  the  4th  lumbar  vertebra,  a  little  behind  the  origin  of  the  spermatics, 
but  this  relationship  may  be  reversed. 

The  External  Iliac  Artery  is,  speaking  generally,  the  vessel  of  supply 
to  the  hind  limb.  It  is  regarded  as  a  terminal  branch  of  the  aorta,  and 
it  has  its  root  at  the  body  of  the  5th  lumbar  vertebra.  It  descends  with 
a  curved  course  at  the  pelvic  inlet,  and  at  the  anterior  border  of  the  pubis 
it  is  directly  continued  as  the  femoral  artery.  It  is  placed  immediately 
beneath  the  peritoneum,  and  each  is  related  on  its  outer  side  to  the 
psoas  parvus,  sartorius,  and  iliacus  muscles,  tlie  tendon  of  the  first  of  these 
separating  it  from  the  great  crui*al  nerve.  On  the  inner  side  it  is  related 
successively  to  the  common  iliac  and  external  iliac  veins,  the  foimer 
separating  it  from  the  internal  iliac  artery.     Its  branches  are: — 

1.  The  Circumflex  Iliac  Artery.  This  is  a  large  artei'y  detached 
from  the  outer  side  of  the  external  iliac  close  to  its  origin.  It  passes 
outwards  across  the  psoas  muscles  ;  and  at  the  outer  edge  of  the  psoas 
magnus  it  divides  into  an  anterior  and  a  posterior  branch.  The  former 
is  distributed  in  the  flank,  beneath  the  internal  oblique  muscle;  and  the 
latter  perforates  the  oblique  muscles  near  the  bony  angle  of  the  haunch, 
and  descends  to  the  thigh. 

2.  The  Artery  of  the  Cord  (Plate  44).  This  is  a  slender  vessel 
arising  close  to  the  preceding,  or  it  may  come  from  the  aorta  itself. 


DISSECTION    OF    THE    ABDOMEN.  323 

Parallel  and  internal  to  the  spermatic  artery,  it  passes  to  the  internal 
abdominal  ring,  and  is  distributed  to  the  spermatic  cord. 

In  the  mare  it  is  represented  by  the  uterine  artery.  This,  which  is  a 
much  larger  vessel,  passes  between  the  layers  of  the  broad  ligament  to 
reach  the  uterus,  being  distributed  to  the  body  of  that  organ,  and 
anastomosing  anteriorly  with  the  uterine  branch  of  the  ovarian  artery, 
and  posteriorly  with  the  vaginal  artery. 

3.  The  Prepubic  Artery  (Plates  45  and  46).  This  vessel  arises  at 
the  anterior  border  of  the  pubis,  and  marks  the  limit  of  the  iliac  and 
femoral  arteries.  It  forms,  at  its  oi*igin,  a  short  common  trunk  with  the 
deep  femoral  artery.  It  is  about  two  or  three  inches  in  length,  and  it 
passes  on  the  anterior  face  of  Poupart's  ligament  to  the  posterior  edge  of 
the  internal  oblique,  where,  at  the  inner  side  of  the  internal  abdominal 
ring,  it  divides  into  the  posterior  abdominal  and  external  pudic  arteries. 

The  Internal  Iliac  Artery.  This  vessel  may  be  described  as  the 
vessel  for  the  supply  of  the  pelvic  walls  and  contents.  It  will  be 
described  with  the  pelvis. 

The  Posterior  Vena  Cava  (Plates  43,  44,  and  45).  This  great  venous 
trunk  is  formed  to  the  right  of  the  termination  of  the  aorta,  by  the  union 
of  the  two  common  iliac  veins.  It  passes  forwards  along  the  right  side 
of  the  lumbar  vertebral  bodies,  until  it  reaches  the  upper  border  of  the 
liver.  Here  it  descends  in  the  anterior  fissure  of  the  liver,  being 
included  between  that  organ  and  the  diaphragm.  Passing  through  the 
foramen  dextmm,  it  enters  the  thorax.  It  is  related  on  its  left  side  to 
the  aorta;  and  on  its  right  to  the  psoas  parvus,  ureter,  kidney, 
suprarenal  capsule,  and  lumbar  sympathetic  cord  of  the  same  side.  It 
receives  the  following  branches : — 

1.  L^imbar  Veins,  exactly  corresponding  to  the  arteries. 

2.  Phrenic  Veins,  or  sinuses  (2).  These  begin  in  the  muscular  rim 
of  the  diaphragm,  and  converge  to  the  foramen  dextrum,  where  they 
join  the  vena  cava.  They  are  distinctly  visible  Avithout  dissection  in 
the  tendinous  centre  of  the  diaphrag-m  (Plate  45). 

3.  Spermatic  Veins  {Ovarian  in  the  mare).  The  right  and  left  veins 
often  unite  before  joining  the  vena  cava. 

4.  Renal  Veins,  the  left  longer  than  the  right. 

5.  Hepatic  Veins.  These  join  the  vena  cava  while  it  lies  in  the 
anterior  fissure  of  the  liver.  They  discharge  the  blood  of  the  portal 
system  of  veins,  after  it  has  circulated  in  the  liver. 

Iliac  Veins.  There  are  external  and  internal  iliac  veins,  with  branches 
coiresponding  in  all  respects  to  the  divisions  of  the  homonymous  arteries. 
The  external  and  internal  iliac  veins  of  each  side,  however,  unite  and 
form  a  short  trunk  termed  the  common  iliac  vein,  which  is  placed  in  the 
angle  of  separation  between  the  external  and  internal  iliac  arteries.  The 
right  and  left  common  iliac  veins  unite  to  form  the  posterior  vena  cava. 


324  THE  ANATOMY  OF  THE  HORSE. 

The  Receptaculum  Chyli.  Separate  the  aorta  and  vena  cava  at  the 
origin  of  the  anterior  mesenteric  artery,  and  look  above  them  for  this. 
It  is  the  diUited  commencement  of  the  thoracic  duct.  It  is  formed  by 
the  union  of  a  variable  number  of  large  lymphatic  vessels,  and  it  is 
continued  forwards  by  the  duct.  This  is  a  thin-walled  vessel  of  small 
calibre  which  passes  into  the  thorax  between  the  pillars  of  the  diaphragm, 
being  generally  to  the  right  of  the  aorta. 

Last  Dorsal  and  First  Two  Lumbar  Nerves  (Plates  44  and  45). 
The  inferior  primary  branches  of  these  nerves  appear  at  the  outer  edge 
of  the  psoas  magnus,  the  last  dorsal  being  close  behind  the  last  rib,  and 
the  other  two  issuing  in  series  behind  it.  These  nerves  have  already  been 
followed  in  the  dissection  of  the  abdominal  wall,  where  they  are  distri- 
buted in  the  region  of  the  flank  to  the  abdominal  muscles,  panniculus, 
and  skin.  These  lumbar  nerves  also  furnish  cutaneous  branches  to  the 
inside  and  front  of  the  thigh. 

3rd  Lumbar  Nerve  (Plates  44  and  45).  The  inferior  primary  branch 
of  this  nerve  will  be  found  in  front  of  the  circumflex  iliac  artery, 
emerging  from  between  the  psoas  magnus  and  parvus  muscles,  after 
having  penetrated  the  substance  of  the  latter.  It  accompanies  the 
posterior  division  of  the  circumflex  iliac  artery  to  the  front  of  the  thigh, 
where  it  is  expended  in  cutaneous  branches.  Before  it  emerges,  it  gives 
branches  to  the  psoas  and  quadratus  muscles. 

Inguinal  Nerves.  There  is  considerable  variation  in  the  mode  of 
formation  of  these,  but  that  figui-ed  in  Plates  44  and  45  is  probably  as 
common  as  any  other.  A  nerve  is  there  seen  passing  obliquely  backwards 
over  the  circumflex  iliac  artery.  It  is  formed  by  the  union  of  two 
branches  which  emerge  at  the  inner  side  of  the  psoas  paiTus,  these  being 
from  the  2nd  and  3rd  lumbar  nerves  respectively.  It  divides  into  three 
sets  of  branches,  viz. : — 

1 .  Muscular,  to  the  internal  oblique. 

2.  Cremasteric,  to  the  cremaster  muscle. 

3.  Inyuinal,  which  descend  in  the  inguinal  canal  to  supply  the 
scrotum,  prepuce  (mammary  gland  in  the  female),  and  surrounding  skin. 

The  Lumbo-sacral  Plexus  (Plate  48).  This  is  the  plexus  of  nerves 
for  the  supply  of  the  hind  limb.  Like  the  corresponding  brachial  plexus, 
the  inferior  primary  branches  of  five  nerves  compose  it,  viz.,  the  4th, 
5th,  and  6th  lumbar,  and  the  1st  and  2nd  sacral  nerves.  There  is  a 
loop  of  communication  between  the  first  of  those  and  the  3rd  lumbar, 
which  to  that  extent  also  enters  into  the  formation  of  the  plexus.  The 
majority  of  its  branches  fall  to  be  dissected  with  the  pelvis,  and  a 
complete  account  will  then  be  given  of  it  (page  349).  In  the  meantime 
only  the  most  anterior  of  its  branches  will  be  dissected. 

1 .  Branches  to  thcpsoas  magnus  and  iliacus.  These  are  derived  from  the 
4th  lumbar  root  of  the  plexus,  or  from  the  loop  between  that  and  the  3rd. 


DISSECTION    OF    THE    ABDOMEN.  325 

2.  The  Anterior  or  ffveat  crural  nerve  (Plate  45).  This  is  a  large 
nei've  which  derives  its  fibres  from  the  4th  and  5th  lumbar  roots,  and 
from  the  loop  between  the  3rd  and  4th.  Emerging  between  the  psoas 
magmis  and  parvus,  it  descends  at  the  outer  side  of  the  external  iliac 
artery,  but  separated  from  it  by  the  tendon  of  the  last-named  muscle. 
It  rests  on  the  iliacus  and  psoas  muscles,  and  crosses  their  common 
termination  to  end  in  a  fascicuhis  of  branches  for  the  extensors  of  the  leg. 
In  this  course  it  is  covered  by  the  sartorius  muscle.  It  gives  off  as  a 
branch  the  internal  saphenous  nerve,  whose  origin  is  aboiit  opposite  the 
ilio-pectineal  eminence. 

The  Aortic  Plexus  of  the  Sympathetic  Nerve  (Plate  45).  This  is  the 
backward  continuation  of  the  solar  plexus.  Its  branches  interlace  around 
the  aorta  behind  the  kidneys,  and  unite  with  the  posterior  mesenteric 
plexus.     It  receives  some  of  the  efferent  filaments  of  the  lumbar  ganglia. 

The  Posterior  Mesenteric  Plexus  is  formed  around  the  root  of  the 
artery  of  the  same  name.  It  is  united  in  front  with  the  aortic  plexus, 
and  receives  efferent  branches  from  the  lumbar  ganglia.  Three  sets  of 
branches  pass  from  it: — 

1.  Branches  following  the  divisions  of  the  posterior  mesenteric  artery. 

2.  Branches  accompanying  the  spermatic  artery,  and  forming  the 
sjyerniatic  2)lexus. 

3.  Pelvic  branches  to  join  the  pelvic  2)lexns. 

The  Sympathetic  Gangliated  Cord  in  the  loins.  This  is  the  back- 
ward continuation  of  the  dorsal  cord.  Beginning  between  the  psoas 
parvus  and  the  diaphragmatic  cms,  it  extends  backwards  to  the  lumbo- 
sacral articulation,  Avhere  it  is  directly  continued  by  the  sacral  division 
of  the  cord.  Each  nerve  will  be  found  on  the  inner  aspect  of  the  psoas 
parvus  muscle  of  the  same  side,  the  left  nerve  being  related  inwardly  to 
the  aorta;  and  the  right  for  the  greater  part  of  its  course  to  the  vena 
cava,  but  for  a  short  distance  in  front  to  the  aorta.  Six  fusiform, 
greyish  ganglia  stud  the  cord,  and  from  these  proceed  the  various 
branches  of  the  cord.     These  branches  are  : — 

1.  Communicating  branches  with  the  inferior  primary  divisions  of  the 
lumbar  spinal  nerves. 

2.  Branches  to  the  aortic  and  posterior  mesenteric  plexuses. 

Iliac  Fascia.  This  is  the  name  given  to  the  aponeurotic  layer  which 
covers  the  inferior  face  of  the  psoas  magnus  and  iliacus  muscles.  It  is 
densest  and  most  adherent  at  the  side  of  the  pelvic  inlet,  and  becomes 
more  cellular  as  it  is  traced  forwards  and  backwards.  It  is  adherent 
inwardly  to  the  psoas  parvus  tendon,  and  outwardly  to  the  bony  promi- 
nence of  the  haunch.  Poupart's  ligament  is  adherent  to  its  inferior  f\ice, 
and  it  gives  origin  to  the  sartorius  and  cremaster  muscles. 

The  Psoas  Magnus  (Plates  44  and  45).  This  muscle  is  broad  and 
flattened  anteriorly,  and  thick  and  pointed  behind,  where  it  rests  in 


32G  THE  ANATOMY  OF  THE  HORSE. 

a  dei:)i"essiou  of  the  iliacus.  It  omes  from  the  last  two  dorsal  vertebrte 
and  the  under  surfaces  of  the  last  two  ribs  at  their  upper  part ;  also 
from  the  lumbar  vertebrse  except  the  last,  covering  their  transvei-se 
processes.  It  is  inserted,  in  common  with  the  iliacus,  into  the  internal 
trochanter  of  the  femur. 

Action. — It  is  a  flexor  and  an  outward-rotator  at  the  hip-joint.  When 
the  hind  limbs  are  fixed,  the  two  muscles  will  arch  the  loins,  or  the  single 
muscle  will  incline  the  trunk  to  the  same  side. 

The  Psoas  Parvus  (Plate  45).  This  is  a  smaller  and  more  tendinous 
muscle  than  the  preceding,  to  whose  inner  side  it  is  placed.  It  arises 
from  the  bodies  of  the  last  three  or  four  dorsal  and  all  the  lumbar 
vertebra?.  It  is  inserted  into  the  ilio-pcctineal  eminence  (of  the  ilium) 
at  the  side  of  the  pelvic  inlet. 

Action. — To  flex  the  pelvis  on  the  loins  when  both  musles  act;  or  to 
incline  it  laterally  when  a  single  muscle  acts.  If  the  pelvis  be  fixed,  it 
will  execute  the  same  movements  on  the  loins. 

Directions. — Raise  the  outer  edge  of  the  psoas  magnus,  and  remove  it 
except  its  fibres  of  origin  beneath  the  rim  of  the  diaphragm  and  its 
conical  tendon  of  insertion,  as  in  Plate  45. 

The  Iliacus  (Plates  45  and  16).  This  is  a  powerful  fleshy  muscle 
which,  when  the  psoas  magnus  muscle  is  in  position,  appears  to  consist 
of  an  outer  and  an  inner  portion.  In  reality,  however,  it  is  a  single 
mass,  with  a  deep  groove  in  it  for  the  terminal  tendon  of  the  psoas 
magnus.  It  ai'ises  from  the  entire  iliac  surface  of  the  ilium,  from  its 
external  angle,  and  from  the  sacro-iliac  ligament.  It  is  inserted  into  the 
inner  trochanter  of  the  femur,  in  common  with  the  psoas  magnus. 

Action. — The  same  as  the  psoas  magnus. 

The  QuADRATUS  LuMBORUM  (Plate  45).  This  muscle  lies  under  cover 
of  the  great  psoas  muscle.  Its  most  external  and  strongest  fasciculus 
arises  from  the  sacroiliac  ligament.  It  is  inserted  by  this  same  fasciculus 
into  the  tips  of  the  lumbar  transverse  processes,  and  into  the  hinder 
edge  of  the  last  rib.  From  the  main  fiisciculus  others  pass  inwards  to 
the  lumbar  transverse  processes,  and  to  the  under  surfaces  of  the  thi-ee 
last  ribs,  close  to  the  spine. 

Action. — To  assist  in  bending  the  loins  to  the  side  of  the  acting  muscle. 
Both  muscles,  by  fixing  the  last  ribs,  will  enable  the  diaphragm  to  act 
to  more  advantage. 

Lumbar  Intertransverse  Muscles  (Plate  45).  These  are  thin 
muscular  and  tendinous  strata  connecting  the  edges  of  adjacent  lumbar 
transverse  processes. 

Action. — To  assist  in  bending  the  loins  to  the  side  on  which  the 
muscles  act. 

Directions. — An  examination  of  the  dia[)hragm  will  complete  the 
dissection  of  the  abdomen.     Define  its  pillars  attaching  it  to  the  lumbar 


DISSECTION    OF   THE    ABDOMEN.  327 

vertebrae,  and  clean  the  edges  of  its  foramina.  At  its  periphery,  under 
the  costal  cartilages,  follow  the  asternal  vessels  (Plate  45). 

The  Asternal  Artery.  This  is  one  of  the  divisions  of  the  internal 
thoracic  artery  (Fig.  7,  page  120).  It  passes  from  the  thorax  to  the 
abdomen  by  perforating  the  rim  of  the  diaphragm  about  the  9th  chondro- 
costal  joint.  As  here  seen,  it  passes  backwards  at  the  rim  of  the 
diaphragm,  in  the  interval  between  it  and  the  origin  of  the  transversalis 
abdominis,  and  terminates  at  the  lower  extremity  of  the  13th  intercostal 
space.  It  has  three  sets  of  branches,  viz.,  (1)  ascending  branches,  which 
anastomose  with  the  intercostal  arteries  of  the  spaces  crossed ;  (2)  internal 
branches  to  the  diaphragm ;  (3)  descending  branches,  which  run  on  the 
peritoneal  surface  of  the  transversalis  abdominis  muscle. 

The  Asternal  Vein  accompanies  the  artery. 

The  Diaphragm  (Plate  45)  is  the  muscle  which  serves  as  a  partition 
between  the  thoracic  and  abdominal  cavities.  In  outline  it  has  some 
resemblance  to  the  heart  of  playing-cards,  the  point  being  at  the  ensiform 
cartilage,  and  the  base  at  the  spine.  Its  general  direction  is  oblique 
downwai'ds  and  forwards.  Its  anterior  or  thoracic  surface  (Plate  22)  is 
convex,  covered  by  pleura,  and  related  to  the  bases  of  the  lungs.  Its 
posterior  surface  is  concave,  covered  for  the  greater  part  by  peri- 
toneum, and  related  to  the  liver,  stomach,  spleen,  and  intestines.  It 
consists  of  a  muscular  rim,  two  muscular  pillars,  or  crura,  and  an 
aponeurotic  centre. 

The  Fleshy  Rim  is  composed  of  soft  muscular  fibres,  the  lowest  of  which 
are  attached  outwardly  to  the  upper  face  of  the  ensiform  cartilage  about 
one  inch  behind  its  junction  with  the  sternum  (Fig.  7,  page  120).  From 
this  mid  point  the  line  of  attachment  of  the  rim  rises  on  each  side,  the 
fibi'es  taking  origin  from  the  cartilages  of  the  last  ten  ribs,  or  from  the 
ribs  themselves  above  the  chondro-costal  joints.  On  each  side  these 
fibres  meet,  or  are  separated  by  only  a  narrow  line  from,  the  fibres  of 
the  transversalis  abdominis  at  their  origin  ;  and  along  the  line  of  separa- 
tion the  asternal  arterj-  runs.  The  muscular  fibres  are  all  directed  from 
these  points  of  origin  inwards,  where,  along  a  denticulated  line,  they 
terminate  in  the  tendinous  centre. 

The  Pillars,  or  Ci'wa. — These  are  right  and  left.  The  right  is  the 
largest,  and  arises  by  a  strong  tendon  from  the  lumbar  vertebrae,  through 
the  medium  of  the  inferior  common  ligament.  Its  muscular  fibres 
terminate  in  the  tendinous  centre,  some  of  them  diverging  to  the  right, 
but  without  joining  the  muscular  rim,  while  others  descend  to  near  the 
mid  point.  The  left  pillar  has  a  similar  origin  from  the  left  side  of  the 
lumbar  vertebrae,  and  its  fibres  terminate  in  the  tendinous  centre,  being 
sometimes  continuous  outwardly  with  the  muscular  rim. 

The  Tendinous  Centre  is  pearl}^  white,  and  composed  of  glistening- 
fibres  interlacing  in  various  directions.     By  the  descent  of  the  pillars 


328  THE    AXATOMY    OF    THE    H()U>;E. 

into  it,  it  is  partially  divided  into  right  and  left  halves,  or  leaflets. 

Ligamenta  Arcuata. — On  each  side  of  the  pillars  the  rim  of  the 
diaphragm  arches  with  a  free  edge  over  the  apices  of  the  psoas  muscles, 
forming  the  so-called  arcuate  ligament. 

Foramina  of  the  diaphragm. 

The  Foramen  Sinistrum. — This  is  a  slit  between  the  fibres  of  the  right 
ci-us,  formed  slightly  to  the  left  of  the  mesial  plane,  and  a  little  below 
the  spine.  It  transmits  the  oesophagus,  the  oesophageal  continuations  of 
the  vagus  nerves,  and  the  pleuro-oesophageal  branch  of  the  gastric  artery. 

The  Foramen  Dextrum. — This  is  the  aperture  by  which  the  posterior 
vena  cava  passes  from  the  abdomen  to  the  thorax.  It  is  formed  near 
the  middle  of  the  tendinous  centre,  but  a  little  to  the  right  of  the  mesial 
plane.  The  margins  of  the  opening  are  closely  adherent  to  the  wall  of 
the  vein,  and  here  the  phrenic  sinuses  empty. 

The  Hiatus  Aortiais. — This  is  the  opening  between  the  right  and  left 
pillars,  close  to  the  spine.  It  gives  passage  to  the  posterior  aorta,  and 
to  the  initial  portions  of  the  thoracic  duct  and  great  azj-gos  veins. 

Between  the  crus  and  the  psoas  parvus  on  each  side  the  gangliated 
cord  of  the  sympathetic  passes,  and  a  little  outward  the  great  splanch- 
nic nerve  passes  between  the  same  muscle  and  the  edge  of  the 
diaphragm.  The  asternal  vessels  penetrate  the  edge  of  the  diaphragm 
at  the  9th  chondro-costal  joint. 

Action  of  the  Diaphragm. — The  diaphragm  is  the  principal  muscle  of 
inspiration.  When  it  contracts,  it  moves  backwards,  and  thus  increases 
the  antero-posterior  diameter  of  the  thorax.  In  this  action  it  pushes 
back  the  abdominal  viscera,  and  causes  the  abdominal  wall  to  descend. 
The  movements  of  the  diaphragm  affect  principally  its  periphery,  any 
great  backward  movement  of  the  tendinous  centre  being  prevented  by 
the  posterior  vena  cava,  Avhich  passes  like  a  ligament  between  the  centre 
and  the  heart 

STRUCTURE    OF    THE    STOMACH. 

The  wall  of  the  stomach  compi-ises  four  layers,  viz.,  serous,  nuiscular, 
submucous,  and  mucous. 

1.  The  Serous  Coat  is  a  smooth,  glistening  covering  derived  from  the 
eritoneum.     It  is  united  to  the  subjacent  muscular  coat  by  areolar 

tissue  sometimes  termed  the  suhserovs  coat. 

2.  The  Muscular  Coat. — This  can  be  best  displayed  on  a  stomach 
which  has  been  boiled  for  a  few  minutes.  If  two  such  stomachs  can  be 
procured,  one  of  them  should  be  everted  and  moderately  inflated,  and 
then  its  mucous  coat  stripped  off  with  fingers  and  forceps.  From  tlie 
other,  similarly  inflated,  the  peritoneum  should  be  stripped  off.  The 
muscular  fibres  are  disposed  in  three  planes,  viz.,  an  outer  longi- 
tudinal, a  middle  circular,  and  an  inner  oblique  layer.       Of  these  the 


DISSECTION    OF    THE   ABDOMEN. 


329 


circular  layer  is  found  all  over  the  organ,  but  the  other  layers  are 
mainly  confined  to  the  left  half  At  the  right  extremity  of  the  stomach 
the  circular  fibres  are  aggregated  to  form 
the  sphincter-like  j^yloric  ring.  The  fibres 
are  of  the  non-striped  variety. 

3.  The  Submucous  Coat  is  composed  of 
areolar  connective-tissue,  in  which  the 
blood-vessels  ramify  before  they  pass  into 
t  he  next  coat. 

4.  The  Mucous  Coat. — It  is  desirable  to 
study  this  on  the  stomach  of  an  animal 
recently  killed.  If  possible,  take  such  a 
stomach  with  about  a  foot  of  the  duo- 
denum and  a  few  inches  of  the  oeso- 
phagus attached,  and  fasten  the  duodenum 
to  a  tap.  Let  water  flow  into  the  organ, 
and  it  will  be  noticed  that,  even  when 
the  stomach  is  much  distended,  none  of 
the  water  escapes  by  the  orifice  of  the 
gullet,  although  that  is  unligatured.  This 
is  an  instructive  experiment,  as  showing 
the  difficulty  or  impossibility  of  vomition  in 
the  horse.  Now  allow  the  contents 
of  the  stomach  to  escape  by  the  duo- 
denum 

convex  curvature.  It  will  at  once  be  noticed  that  the  mucous  lining  is 
not  the  same  throughout.  The  left  or  cardiac  half  of  the  cavity  is  lined 
by  a  mucous  membrane  termed  cuticular  ;  the  right  or  pyloric  half  has 
a  totally  diff"erent  lining,  termed  villozis.  The  cuticular  portion  is  pale, 
harsh,  without  true  gastric  glands,  but  possessed  of  a  few  mucous  follicles, 
and  covered  on  its  free  surface  by  a  thick  stratified  squamous  epithelium. 
It  is,  in  fact,  an  extension  of  the  oesophageal  mucous  membrane,  which 
it  resembles  in  all  respects.  Towards  the  middle  of  the  stomach  it  is 
separated  from  the  villous  half  by  an  abrupt,  raised,  and  slightly  sinuous 
line  of  demarcation — the  cuticular  ridge.  The  villous  half  is  rosy,  soft, 
and  velvety  (but  without  villi),  thickly  beset  with  gastric  glands,  and 
possessed  of  a  single  layer  of  columnar  epithelium.  The  gastric  glands 
are  of  the  tubular  variety,  and  by  the  aid  of  a  lens  numbers  of  them 
may  be  seen  opening  together  into  pits,  or  alveoli^  of  the  mucous  mem- 
brane. The  cuticular  portion  is  but  slightly  vascular,  but  the  villous 
portion  is  richly  supplied  with  blood-vessels.  In  the  collapsed  organ 
the  mucous  membrane  is  thrown  into  folds,  or  riigce. 

The  GEsoPHAGEAL  Orifice,  it  will  now  be  seen,  is  very  nan-ow,  and 
obstructed  by  the  mucous  membrane  gathered  into  folds. 


Fig.  38. 
Vertical    Transverse   Section   of 
THE  Coats  of  a  Pig's  Stomach. 
30  Diameters  (from  Kblliker). 
a.  Gastric  glands  ;  6.  Muscular  layer 
of  the  mucous  membrane  ;    c,   Sub- 
mucous or  areolar  coat;   d.  Circular 
muscular  layer ;  e.  Longitudinal  mus- 
cular layer  ;  /.  Serous  coat. 

and  either  evert  the  organ  and  inflate  it,  or  incise  it  along  its 


330 


THE   ANATOMY    OF   THE    HORSE. 


The  Pyloric  Orifice  is  much  larger,  but  capable  of  being  completely 
closed  by  the  pyloric  ring  of  muscular  fibres. 


Fig. 

Stomach,  everted  and  inflated. 
1.  Left  (cardiac)  sac  vith  its  eutkular  mucous  lining;   2.  Right  (pyloric)  sac  with   its  rillous 
mucous  lining ;  3.  Cutieulav  ridge  ;  4.  Termination  of  ojsophagus  ;  5.  Initial  part  of  duodenum  ; 
6.  Pyloric  ring. 

In  the  interior  of  the  duodenum,  about  six  inches  from  the  pyloinis, 
the  openings  of  the  bile  and  pancreatic  ducts  will  be  found.  The  orifices 
of  the  bile  duct  and  duct  of  Wii-sung  are  placed  together  on  the  con- 
cave side  of  the  bowel,  and  are  surrounded  in  common  by  a  ring-like 
valvular  fold  of  mucous  membrane.  The  opening  of  the  accessory  pan- 
creatic duct  is  placed  opposite  to  these. 

STRUCTURE    OF    THE    LIVER. 

Lay  the  organ  with  its  diaphragmatic  surface  downwards.  Find  the 
portal  vein,  hepatic  artery,  and  bile  duct,  at  the  portal  fissure,  and  trace 
them  for  a  little  distance  into  the  liver.  Invert  the  organ,  and  obser\'e 
the  coui-se  of  the  anterior  vena  cava  in  the  anterior  fissure,  and  the  mouths 
of  the  hepatic  veins  which  there  discharge  themselves  into  the  cava. 

Tunics  or  Capsules  of  the  liver.  These  are  two  in  number :  LA 
peritoneal  coat,  giving  the  free  surface  of  the  organ  its  smooth  and 
glistening  characters.  2.  A  tunica  pi-opria,  or  ^fibrous  coat,  placed  beneath 
the  preceding.  All  over  the  surfiice  of  the  liver  it  sends  inwards  delicate 
processes  that  join  the  interlobular  connective-tissue,  and  at  the  portal 


DISSECTION    OF    THE    ABDOJIEN. 


331 


fissure  it  furnishes  a  sheath  that  accompanies  the  poi-tal  vein,  hepatic 
artery,  and  bile  duct  into  the  liA'er.     This  sheath  is  the  capsule  of  Glisson. 

Lobules  of  the  Liver. — When  a  fresh-cut  surface  of  the  liver  is  examined, 
it  shows  a  system  of  lines  mapping  it  out  into  areas  about  the  size  of  a 
pin's  head.  These  areas  are  sec- 
tions of  the  lobules  of  the  liver, 
which  are  united  together  by 
interlobular  connective  -  tissue. 
This  interlobular  connective-tissue 
is  much  more  abundantly  de- 
veloped in  the  pig,  and,  conse- 
quently, in  that  animal  the  lobu- 
lation of  the  liver  substance  is 
much  more  evident.  A  lobule 
may  be  viewed  as  having  a  frame- 
work of  blood-vessels,  in  which  are 
set  the  liver-cells.  Between  the 
adjacent  cells  the  rootlets  of  the 
bile  passages  begin,  and  there  are 
possibly  also  branches  of  nerves 
and  lymphatic  vessels. 

The  liver  is  supplied  wdth  blood 
by  two  vessels.      The    first   and 

much  the  larger  of  the  two  is  the  longitudinal    section    of    a    Portal    Canal, 

Tinrtnl  vpin    tlip  nthpr   i«  flip  Vipn-  containing  a  Portal  Vein,  Hepatic  Artery, 

portal  \eni,  tne  Otnei    is  ine  nep-  aj,d   Hepatic    Duct,    from    the    Pig    (after 

atic  arterv  Kleman).     about  5  diameters. 

mi        T.                  17                11      X      -i.  P-  Branch  of  vena  portiB,  situated  in  a   portal 

ihe    rORTAL     VEIN    collects    its  canal,  formed  amongst  the  hepatic  lobules  of  the 

blood  from  thestomach,  intestines,   ^ZlJL^'c^:^ iT^a'i^^I^!^^  f^ 
spleen,  and  pancreas.     Entering   ^Z.'^f^ J'^r^nl^K^rir^l^ 

the  liver  at  the  portal  fissure,  this    ;j;;^^f' ^  ^"^"'^  '*  •   "■  ^epatio  artery  ;    ,/.   Biliary 

vein  comports  itself  like  an  ar  tery, 

in  that  it  reduces  itself  by  division  and  subdivision  to  branches  that 
■come  progressively  smaller  until  they  terminate  in  a  set  of  capillaries. 
In  their  course  through  the  liver,  the  larger  branches  of  the  vein  run 
in  tunnels  of  the  liver  substance — the  portal  canals — which  contain 
also  branches  of  the  hepatic  artery  and  bile  ducts,  and  are  lined  by 
Glissou's  capsule.  The  smaller  branches  of  the  portal  vein  are  distri- 
buted in  the  interlobular  connective-tissue,  where,  at  the  circumference 
\.f  each  lobule,  they  form  an  interlobular  jilexiis.  From  this  plexus 
capillary  vessels  penetrate  the  lobule,  and  form  within  it  the  intra- 
lobular 'plexm.  The  capillaries  of  this  last  plexus  converge  towards 
the  axis  of  the  lobule,  and  there  empty  themselves  into  what  is  termed 
the  central  vein  of  the  lobule.  This  is  the  initial  vessel  of  the  hepatic 
system  of  veins,  and  at  the  base  of  the  lobule  it  joins  a  larger  vessel — 


332 


THE    AXATOMY    OF    THE    HORSE. 


tlie  siihlolndar  vein.  By  the  xiuiou  of  these  sublobular  veins  thi-oiigh- 
<jut  the  liver,  the  lai-ger  hepatk  venous  trunks  are  foi-med ;  and  these, 

as  ah-eadj  seen,  enter  tlie  pos- 
torit)r  vena  cava  in  the  anterior 
tissure  of  the  liver. 

The  Hepatic  Artery  is  a 
branch  of  the  coiliac  axis.  It 
enters  the  liver  with  the  portal 
vein,  and  ramifies  with  it.  It 
has  three  sets  of  branches  :  (1) 
(xqjsida)'  branches,  to  the  tunica 
propria ;  (2)  vaf/inal  branches, 
to  Glisson's  capsule  and  the 
vessels  within  it ;  and  (3)  inier- 
^^^-  41-  lobular   branches,    whose     capil- 

Traxsverhe  Section  through  the  Hepatic  Lobules  laries     paSS      intO      the     lobulc, 

i,  i,  i.  Interlobular  veinrending  in  the  intralobular  ''"''^^^^'^  t^^^y  help  tO  fomi  the 
capillaries;  c,  c.  Central  veins  joined  by  the  intra-  in+vilnhiilnr  ii1pyii«!  •mrl  mi+pv 
lobular  capillaries.  At  a,  o.  the  capillaries  of  one  l»tialODUlai  piCXUS,  ana  eutei 
lobule  coniniunicate  with  those  adjacent  to  it.  the     Central      Vein.       TllC     Capil- 

laries of  the  vaginal  and  capsular  branches  terminate  in  veins  that  join 
the  portal  vessels. 

The  Liver  Cells. — These  are  granular  nucleated  masses  of  protoplasm, 
often  containing  fat  particles.  They  are  arranged  in  columns  between 
the  strands  of  the  intralobular  plexus  of  capillaries. 

The  Bile  Passages  begin  within  the  lobule  as  a  network  of  fine  canals 
— the  bile  capillaries — tunnelled  at  the  lines  of  apposition  of  the  liver 
cells.  At  the  periphery  of  the  lobule  these  become  continuous  with 
interlobular  bile  ducts  having  a  proper  wall  and  a  simple  columnar 
epithelial  lining.  The  interlobular  bile  ducts  unite  to  form  the  larger 
ducts  that  accompany  the  blood-vessels  in  the  portal  canals,  and  these 
finally  form  the  main  bile  duct,  which  passes  in  the  gastro-hepatic 
omentum  to  perforate  the  wall  of  the  duodenum. 


structure  of  the  spleen. 


The  spleen,  like  the  liver,  possesses  two  coats,  viz.,  an  outer  serous  or 
2)e7'itoneal  coat,  and  a  deeper  fibrous  tunic,  or  tunica  propria.  The  latter 
is  composed  of  white  fibrous  tissue  with  a  considerable  admixture  of 
elastic  and  non-striped  muscular  fibres.  It  detaches  from  its  inner  sur- 
face a  nuiltitude  of  trabeadce,  which  by  their  anastomosis  ft)rm  a  fibrous 
framework  in  the  interior  of  the  organ.  The  interspaces  of  this  frame- 
work are  occupied  by  a  grumous  material — the  splenic  pulp.  If  the  cut 
surface  of  the  spleen  be  washed  beneath  a  tap,  the  pulp  may  be  removed 
and  the  fibrous  trabecula)  rendered  very  evident. 

The  S2)lenic  Artery,  a  division  of  the  cocliac    axis,  is  a    very  large 


DISSECTION    OF    THE    ABDOMEN. 


333 


vessel.  Its  branches  enter  at  the  concave  border  of  the  spleen,  and 
carry  with  them  sheaths  derived  from  the  fibrous  tunic.  These 
branches  reduce  themselves  by  division, 
and  the  smaller  bi-anches  are  remark- 
able in  having  the  outer  coat  formed  of 
lymphoid  tissue  Here  and  there  this 
lymphoid  tissue  forms  distinct  swellings 
developed  either  uniformly  ai-ound  the 
arteries,  or  more  or  less  to  one  side. 
These  are  the  Malpighicm  bodies  of  the 
spleen.  The  arteries  terminate  in  tufts 
of  capillary  vessels  in  the  pulp.  They 
are  believed  to  have  incomplete  walls, 
allowing  their  contents  to  escape  and 
form  the  pulp.  The  rootlets  of  the 
splenic  vein  begin  in  the  same  manner, 
having  incomplete  walls  through  which 
their  lumen  is  continuous  with  the  spaces 
lodging  the  pulp.  Gradually  their  walls 
become  thicker  and  complete,  and  adja- 
cent veins  uniting  on  their  course  towards 
the  anterior  border  form  the  large  splenic 
vein,  which  is  one  of  the  main  branches 
of  the  portal  vein. 

The  Splenic  Pulp  possesses  a  supporting  network  of  retiform  connective- 
tissue;  and  the  meshes  of  this  network  are  set  with  many  lymphoid 
cells  like  the  colourless  corpuscles  of  the  blood,  and  with  red  blood 
corpuscles,  normal  or  in  different  stages  of  disintegration 


Cut  Surface  of  Horse's  Spleen, 
trabecular  framework. 


STRUCTURE    OF    THE   PANCREAS. 

The  pancreas  is  a  compound  tubular  or  racemose  gland.  It  is  com- 
posed of  lobules  held  together  by  a  connective-tissue  framework.  When 
the  main  ducts  of  the  gland  are  traced  backwards  into  the  gland,  they 
are  found  to  be  formed  by  the  imion  of  smaller  ducts,  and  so  on  until 
the  smallest  ducts  are  reached.  These  begin  in  the  alveoli,  which  are 
lined  by  secretory  epithelium. 


STRUCTURE    OF    THE    KIDNEY. 

The  kidney  is  invested  by  a  fibrous  cajjsule.  In  health  this  can  with- 
out difficulty  be  stripped  off  the  kidney  substance,  to  which  it  is  con- 
nected only  by  delicate  processes  and  vessels.  If  a  horizontal  section  be 
made  from  the  convex  border  to  the  hilus  of  the  kidney,  the  organ  will 
be  seen  to  possess  a  cavity  towards  the  hilus,  termed  the  pe^yw,  and  to 


334  THE  ANATOMY  OF  THE  HORSE. 

consist  of  two  diftercnt  kinds  of  tissue — the  cortical  and  the  medullary 
substance  of  the  kidney. 

The  renal  pelvis  is  a  curved  cavity,  its  extremities  being  termed  the 
arms.  On  its  outer  side  there  is  a  horizontal  ridge — the  renal  a-est — 
on  which  the  uriniferous  tubules  open,  and  on  its  inner  side  it  is  con- 
tinuous by  a  funnel-shaped  opening  with  the  lumen  of  the  lu'eter. 

The  cortical  substance  forms  a  layer  beneath  the  capsule  ;  the  medulla 
is  disposed  around  the  pelvis  and  is  internal  to  the  cortical  substance. 
The  cortex  is  about  twice  as  thick  as  the  medulla,  but  the  two  layers 
meet  along  a  sinuous  line,  and  slightly  intei-penetrate  one  another.  It 
will  be  noticed  that  the  two  layers  contrast  with  one  another  in  the 
following  respects : — The  cortex  is  of  a  deep  red  colour,  it  is  granular, 
friable,  and  studded  with  numerous  small  shining  points — the  Malpigkian 
bodies.  The  medulla,  on  the  other  hand,  is  pale  red,  striated,  and 
fibrous-looking,  less  friable  than  the  cortex,  and  without  any  ^lalpighian 
bodies. 

Uriniferous  tvbules. — The  largest  tubes,  or  papillari/  ducts,  open  on  the 
crest  of  the  pelvis.  If  such  a  tube  be  traced,  it  will  be  found  to  pass 
oxitwards  through  the  medulla,  having  a  straight  course,  and  branching 
dichotomously.  The  smaller  tubes  resulting  from  this  division  are 
called  the  collecting  tubes ;  and,  still  preserving  their  rectilinear  course, 
they  enter  the  cortex  in  bundles  termed  the  pi/ramids  of  Ferrein.  At 
the  surface  of  these  pyramidal  bundles,  the  straight  tubes  curve  out- 
wards in  the  cortex,  and  become  dilated  and  tortiious,  forming  the 
intermediary  or  junctional  tubules.  Each  of  these  is  succeeded  by  a 
narrow  straight  tubule,  which  descends  from  the  cortex  to  the  medulla, 
where  it  forms  a  bend,  or  loop,  and  i-uns  up  again  into  the  cortex. 
There  is  thus  formed  the  loojied  tube  of  Henle,  which  is  shaped  like  the 
letter  U.  Having  re-entered  the  cortex,  Henle's  tube  becomes  dilated 
and  tortuous,  constituting  the  convoluted  tube,  which  becomes  constricted 
and  then  expands  into  a  bladder-like  dilatation — Bowman's  capsule. 
Boivman's  capside  siuTounds  a  clue-like  tuft  of  capillary  vessels  called 
the  glomerulus,  and  the  whole  constitutes  a  Malpighian  body.  It  is 
more  natural,  but  less  simple  at  first,  to  regard  the  tube  as  beginning 
not  at  the  crest  of  the  pelvis,  but  at  Bowman's  capsule.  The  student 
should  mentally  work  it  out  in  that  direction  for  himself  The  urini- 
ferous tubules  consist  of  a  basement  membrane  with  an  epithelial  lining. 
In  the  convoluted  and  intermediary  tubes  the  cells  are  irregularly 
columnar,  but  their  outlines  are  obscure;  in  the  descending  limb  of 
Henle's  tiibe  (neai'est  the  capsule  of  Bowman)  the  cells  are  flattened ; 
and  elsewhere  the  cells  lining  the  tubes  are  cubical  or  columnar. 

The  Renal  Vessels.  The  renal  artery  divides  into  a  number  of 
branches  which  penetrate  the  kidney  near  the  lulus.  Reaching  the 
boundary  line    between  the    cortex    and   medulla,  the  artei'ies   divide 


DISSECTION    OP    THE    ABDOMEN. 


335 


and  anastomose  to  form  a  series  of  arches  from  wliich  both  cortical  and 
medullary  vessels  arise. 

The  cortical  or  interlohtdar 
arteries  are  larger  and  more 
numerous  than  those  for  the 
medulla.  They  pass  directly 
outwards  towax-ds  the  surface  of 
the  kidney,  giving  off  lateral 
branches — the  vasa  afferentia — to 
Bowman's  capsule,  and  terminal 
branches  to  the  fibrous  coat  of 
the  kidney.  Each  vas  afferens 
pierces  Bowman's  capsule,  and 
resolves  itself  into  the  glomer- 
iihis,  or  capillary  tuft.  From 
this  again  the  blood  is  led  out 
of  Bo^\^nan's  capsule  by  the 
vas  efferens.  The  vasa  efferentia 
again  resolve  themselves  into 
capillaries,  and  these  form  a 
network  among  the  convoluted 
tubes.  From  this  intertubular 
capillary  network,  small  veins 
arise  and  pass  to  join  the  inter- 
lobular veins,  ninning  alongside 
the  arteries.  These  interlobular  vj^gg^Ls  of  the  kidxeys 
veins  begin  at  the  surface  of  the 
kidney  by   the    convergence    of  _         .  . 

,  />        •       X  •         c  voluted  tube ;'  0.  Bowman's  capsule ;  A.  Segment  of 

a  number    of  mmute    vemS   from  artery  forming  renal  arch  ;  B.  Interlobular  artery 

+l->o  ^Qnoiilo       fnvniincr  fViP    <ifel1ntp  C.  Afferent  vessel  of  glomerulus;  D.  Efferent  vessel 

tne  capsule— 101  mmg  tne   SteUate  ^^  the  same;   B.  Glomerulus;  F.  Plexus  formed  by 

veins.      The    interlobular    veins  ^^^  ^^^^«°*'''^  5  ^- ^^'^^^''^ '"<='**!  2- ^"^^'■i°^"'^'^ 

join  venous   arches   disposed    in 

the  boundary  layer  between  cortex  and  medulla,  and  from  these  arise 

the  larger  branches  that  finally  unite  to  form  the  large  renal  vein  at  the 

hilus. 

The  medulla  is  less  vascular  than  the  cortex.  Springing  from  the 
arterial  arches  in  the  boundary  layer  are  branches  that  break  up  into 
pencils  of  long  straight  arterioles — the  arteriolce  rectce.  These  pass  with 
a  rectilinear  course  between  the  straight  tubules  of  the  medulla,  and 
break  up  into  a  wide-meshed  capillary  netv.-ork  around  and  between  these 
tubules.  Veins  having  a  straight  course  like  the  arteries  run  in 
company  with  them,  and  join  the  venous  arches  in  the  boundary  layer. 

Connective-tissue  of  the  Kidney. — This  exists  very  sparingly  between 
the  tubes  in  the  cortex,  but  more  abundantly  in  the  medulla. 


Fig.  4.3. 

.  and  uriniferous  tubules 
(motUfled  from  Turner). 
1.  Papillary  duct ;    2.   Collecting  tube  ;    3.  Inter- 
mediary tube ;  4.    Looped  tube   of  Henle  ;  5.    Con- 


336 


THE  ANATOMY  OF  THE  HORSE. 


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DISSECTION   OF   THE   ABDOMEN. 


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CHAPTER    XL 

DISSECTION   OF   THE   PELVIS. 

Under  this  heading  there  will  be  described  not  only  the  pelvic  cavity 
and  its  contents,  but  also  the  tail  and  the  hip-joint. 

Directions. — The  dissection  of  the  abdomen  having  been  completed, 
the  vertebral  column  should  be  sawn  across  or  disarticulated  about  the 
middle  of  the  lumbar  region.  If  the  directions  given  on  page  69  have 
been  attended  to,  the  dissector  of  the  pelvis  should  find  the  hip-joint 
intact,  with  the  femur  sawn  across  below  the  small  trochanter,  as  in 
Fig.  48.  The  muscles  or  portions  of  muscles  left  around  the  hip-joint 
should  be  carefully  removed,  and  the  ligaments  of  the  joint  are  to  be 
dissected,  noticing  in  the  first  place,  however,  its  movements. 

THE   HIP-JOINT   AND    THE   LIGAMENTS    OF    THE    PELVIS. 

The  Hip-joint  belongs  to  the  class  of  enarthrodial  or  ball-and-socket 
joints. 

The  bones  that  enter  into  its  formation  are  the  femur  and  the  os 
innominatum,  the  former  furnishing  a  rounded  hemispherical  head,  and 
the  latter  a  cup-like  cavity — the  acetahidum,  or  the  cotyloid  cavity. 

Movements.  If  the  stump  of  the  fem\ir  be  grasped,  it  will  be  found 
to  have  a  great  freedom  of  movement.  Thus,  it  can  be  flexed,  extended, 
abchicted,  addiicted,  circumducted,  and  rotated.  In  flexion  the  femur  is 
carried  forwards  so  as  to  diminish  the  angle  formed  by  that  bone  and 
the  ilium.  For  the  definition  of  the  other  terms  see  page  42.  In  the 
horse  the  hip-joint  admits  of  a  greater  range  of  movement  than  any 
other  joint  of  the  limbs.  The  movement  of  abduction,  however,  is  less 
free  than  it  is  in  the  other  domestic  animals,  being,  as  will  presently 
be  seen,  restricted  by  the  pubio-femoral  ligament. 

The  joint  possesses  four  ligaments,  viz.,  capsular,  cotyloid,  pubio- 
femoral,  and  round  ligaments. 

The  Capsular  Ligament  has  the  form  of  a  double-mouthed  sac, 
attached,  on  the  one  hand,  to  the  rim  of  the  cotyloid  cavity,  and  to  the 
cotyloid  ligament,  and,  on  the  other  hand,  to  the  periphery  of  the 
articular  head  of  the  femur.  It  is  strengthened  in  front  by  an  oblique 
band  representing  the  ilio-femoral  ligament  of  man.       Its  inner  face  is 


DISSECTION    OF   THE   PELVIS.  339 

lined  by  the  synovial  membrane  of  the  joint,  while  its  outer  face  is 
supported  by  the  following  muscles : — the  deep  gluteus  above,  the 
obturator  extemus  below,  the  rectus  femoris  and  the  rectus  parvus  in 
front,  and  the  gemelli  behind.  The  ligament  should  be  incised  to  show 
the  synovial  membrane,  after  which  it  may  be  removed  entirely. 

The  Synovial  Membrane  forms  a  complete  internal  lining  to  the 
capsular  ligament,  and  also  invests  the  pubio-femoral  and  round  ligaments 
in  the  interior  of  the  joint. 

The  Cotyloid  Ligament  is  a  ring  of  fibro-cartilage  fixed  at  the  margin 
of  the  cotyloid  cavity,  which  it  serves  to  deepen  for  the  reception  of  the 
femoral  head.  On  the  inner  side  of  the  joint,  where  the  notch  interrupts 
the  rim  of  the  cotyloid  cavity,  the  ligament  bridges  over  the  gap,  and  to 
this  portion  of  the  ring  the  term  transverse  ligament  is  sometimes 
applied.  This  portion  of  the  ligament,  thus,  converts  the  notch  into  a 
foramen,  through  which  the  pubio-femoral  ligament  enters  the  joint. 

The  PuBio- Femoral  Ligament.  This  ligament  deriA^es  its  fibres 
from  the  prepubic  tendon  of  the  abdominal  muscles,  the  right  and 
left  ligaments  intercrossing  their  fibres  in  front  of  the  pubes.  It  is 
directed  outwards  and  backwards,  resting  in  a  groove  on  the  inferior 
siu-face  of  the  pubis,  and  perforating  the  origin  of  the  pectineus  muscle. 
At  the  notch  on  the  inner  side  of  the  cotyloid  ligament,  it  enters  the 
hip-joint  by  passing  above  (in  the  natural  position)  the  so-called 
transverse  ligament,  and  it  terminates  in  the  depression  on  the  head  of 
the  femur.  The  ligament,  being  attached  across  the  middle  in  front,  is  put 
upon  the  stretch  Avhen  the  limb  is  abducted,  and  therefore  restricts 
that  movement. 

The  Round  Ligament  (interartimlar  ligament,  or  ligamentum  teres). 
This  short  and  strong  ligament  is  fixed  above  to  the  non-articular 
depression  at  the  bottom  of  the  cotyloid  cavity,  and  below  to  the  excava- 
tion on  the  head  of  the  femur,  being  confounded  at  the  latter  point  with 
the  pubio-femoral  ligament.  It  will  be  best  displayed  by  cutting  the 
transverse  ligament  and  abducting  the  femur. 

Direction. — It  is  convenient  to  dissect  at  this  stage  the  sacro-sciatic 
ligament,  as  it  is  necessary  to  remove  it  in  order  to  display  the  pelvic 

contents.      Along  with  it,  there  will  be  described  two  other  ligaments 

the  superior  and  inferior  ilio-sacral  ligaments. 

The  Sacro-sciatic  Ligament  (Plate  16,  and  Fig.  48).  This  is  a  laro-e 
membranous  ligament  which  forms  the  greater  part  t)f  the  lateral 
boundary  of  the  pelvis.  It  is  irregularly  four-sided  in  form.  Its  upper 
edge,  which  is  pierced  by  the  ischiatic  artery,  is  fixed  to  the  lateral  lip 
of  the  sacrum,  and  to  the  rudimentary  transverse  processes  of  the  first 
one  or  two  coccygeal  bones;  its  lower  edge  is  attached  to  the  superior 
ischiatic  spine  and  to  the  tuber  ischii,  and  between  these  points  it  forms 
the  upper  boundaiy  of  the  small  sacro-sciatic  foramen ;  its  antei'ior  edge 


340  THE  ANATOMY  OF  THE  HORSE. 

is  short,  and  forms  the  posterior  boundary  of  the  great  sacro-sciatic 
foramen;  its  posterior  edge,  much  more  extensive  than  the  anterior,  is 
thin,  ill-defined,  and  iinited  to  the  coccygeal  origin  of  the  semimem- 
branosus. Its  outer  surfixce  is  crossed  by  the  great  sacro-sciatic  nerve, 
and  is  covered  by  the  biceps  femoris  and  semitendinosus  muscles,  which 
in  part  arise  from  it.  Its  inner  surface  is  lined  anteriorly  by  pei'itoneum, 
and  is  related  posteriorly  to  the  compressor  coccygis  and  retractor  ani 
muscles,  some  of  whose  fibres  take  origin  from  it.  The  internal  pudic 
nerve  and  vessels  cross  this  surface,  or  they  may  be  partly  embedded  in 
the  texture  of  the  ligament. 

The  Great  Sacro-sciatic  Foramen  is  an  elliptical  opening  in  the  lateral 
wall  of  the  pelvis,  its  anterior  boundary  being  formed  by  the  ischiatic 
edge  of  the  ilium,  and  its  posterior  by  the  sacro-sciatic  ligament.  It 
transmits  the  gluteal  nerves  and  vessels,  and  the  great  sciatic  nerve. 

The  Sinall  Sacro-sciatic  Foramen  is  an  interval  in  the  lower  and 
posterior  part  of  the  lateral  wall  of  the  pelvis.  Its  upper  edge  is  formed 
by  the  sacro-sciatic  ligament ;  its  lower  by  the  smooth  and  romided 
external  border  of  the  ischium,  between  the  tuber  and  the  superior 
ischiatic  spine.  By  this  opening  the  common  tendon  of  the  obturator 
internus  and  pyriformis  emerge  from  the  pelvis,  and  the  nerves  to  these 
muscles  pass  in. 

The  Superior  Ilio-sacral  Ligament  (Fig.  48)  is  cord-like,  and  passes 
between  the  internal  angle  of  the  ilium  (the  angle  of  the  croup)  and  the 
summits  of  the  sacral  spines. 

The  Inferior  Ilio-sacral  Ligament  (Fig.  48)  is  membranous  and 
triangular  in  form.  Its  anterior  edge  is  fixed  to  the  upper  part  of  the 
ischiatic  border  of  the  ilium ;  its  lower  edge  is  attached  to  the  lateral 
lip  of  the  sacrum ;  its  posterior  or  upper  edge  is  ill-defined,  being  con- 
tinuous with  the  fascia  investing  the  muscles  of  the  tail. 

the  cavity  of  the  pelvis. 

Directions. — Fix  the  pelvis  on  a  table,  with  the  inlet  looking  upwards. 
Sponge  out  the  cavity  and  distend  the  bladder  with  air  or  some  preserv- 
ative fluid,  tying  the  urethra  to  prevent  its  escape. 

The  pelvis  is  not  distinct  from  the  abdominal  cavity,  but  is  merely  a 
backward  continuation  of  it.  It  is,  in  fact,  that  portion  of  the  general 
cavity  of  the  belly  which  is  posterior  to  the  bony  circle  formed  by  the 
sacrum,  pubes,  and  ilio-pectineal  lines.  The  plane  of  separation  between 
the  abdominal  cavity  proper  and  the  pelvic  cavity,  is  termed  the  inlet  of 
the  pelvis ;  the  posterior  extremity  of  the  pelvic  cavity  is  termed  its 
outlet. 

The  inlet  or  brim  of  the  pelvis  is  circumscribed  by  the  promontory  of 
the  sacrum  above,  by  the  anterior  margin  of  the  pubic  bones  below,  and 
by  the  ilio-pectineal  line  on  each  side.     It  looks  downwards  and  forwards. 


DISSECTION    OP    THE   PELV] 


341 


and  it  is  considerably  larger  in  the  mai*e  than  the  horse.  In  form  it  is 
nearly  circular. 

The  outlet  of  the  pelvis  is  circumscribed  by  the  first  one  or  two 
coccygeal  bones  above,  by  the  posterior  edges  of  the  ischial  bones  below, 
and  by  the  posterior  edge  of  the  sacro-sciatic  ligament  on  each  side.  In 
outline  it  is  ovoid,  Avith  the  broad  end  below ;  and  it  looks  backwards 
and  upwards,  being  nearly  parallel  to  the  inlet. 

The  Cavity  of  the  Pelvis  is  the  irregularly  tubular  passage  between 
the  inlet  and  the  outlet.  Its  transverse  section  approaches  the  circular 
in  front,  but  changes  gradually  to  the  oval  as  it  is  taken  more  posteriorly. 
For  convenience  of  description,  however,  it  may  be  said  to  have  a  roof, 
a  floor,  and  two  lateral  walls.  The  roof  is  formed  by  the  inferior  surface 
of  the  sacnim  and  first  one  or  two  coccygeal  bones.  The  floor  is  formed 
by  the  pubic  and  ischial  bones.  Each  lateral  wall  is  formed  for  a  short 
space  in  front  by  the  pelvic  surface  of  the  shaft  of  the  ilium,  and  for  the 
rest  of  its  extent  by  the  sacro-sciatic  ligament. 

Contents  of  the  Cavity. — These  vary  with  the  sex.  In  both  sexes  it 
contains  the  rectum,  the  urinary  bladder,  and  the  termination  of  the 
ureters,  and  numerous  important  vessels  and  nerves.  In  the  male  it 
lodges,  besides  these,  the  vasa  defereutia  (in  part),  the  seminal  vesicles, 
the  prostate,  Cowper's  glands,  the  ejaculatory  ducts,  and  the  prostatic 
and  membranous  portions  of  the  urethra.  In  the  female  it  lodges 
the  posterior  part  of  the  uterus,  the  vagina,  and  the  vulva. 

The  Peritoneum.  The  serous  lining  of  the  abdominal  cavity  is  con- 
tinued into  the  pelvis,  whose  walls  and  contents  it  in  part  covers.  Thus, 
if  it  be  followed  backwards  along  the  roof  of  the  cavity,  it  will  be  seen 
to  cover  the  lower  face  of  the  sacrum  about  as  far  as  its  4th  segment, 
but  at  that  point  it  is  reflected  on  to  the  rectum.  Again,  if  the  peri- 
toneum be  traced  over  the  pelvic  brim  at  the  pubes,  it  will  be  found  to 
cover  the  floor  of  the  pelvis  for  a  short  distance,  and  then  to  become  re- 
flected on  to  the  bladder.  In  the  same  way,  along  a  curved  line  on  the 
side  of  the  pelvis  between  these  two  points,  the  peritoneum  leaves  the 
pelvic  Avail  and  passes  on  to  the  viscera.  Since  this  reflection,  however, 
takes  place  anterior  to  the  posterior  extremity,  or  outlet,  of  the  cavity, 
it  results  that  the  pelvic  viscera  get  at  most  only  a  partial  covering  of 
peritoneum.  Thus,  the  rectum  for  a  length  of  from  four  to  six  inches  in 
front  of  the  anus,  the  posterior  extremity  of  the  vesiculfe  seminales,  and 
(in  the  collapsed  state)  nearly  the  half  of  the  upper  face  of  the  bladder, 
and  three-fourths  of  its  lower  face  are  without  a  serous  covering. 

In  the  mare,  in  the  same  manner,  the  posterior  part  of  the  vagina  and 
the  Avhole  of  the  vulva  are  without  a  serous  covering. 

The  peritoneum  in  passing  on  to  the  viscera  foi-ms  certain  folds,  or 
ligaments.  Thus,  it  forms  below  and  on  each  side  of  the  urinary  bladder 
a  double  fold,  the  inferior  and  lateral  ligaments  of  the  organ  (Plate 


342  TUE  ANATOMY  OF  THE  HORSE. 

44).  The  inferior  lirjament  is  a  mesial  fold  attaching  the  bladder  to  the 
pubic  symphysis,  and  to  the  middle  line  of  the  abdominal  wall  in  front 
of  the  pubic  brim.  The  lateral  ligaments  pass  between  the  sides  of  the 
bladder  and  the  lateral  walls  of  the  pelvis,  and  in  the  adult  the  free 
(anterior)  edge  of  each  contains  the  cord-like  remains  of  the  umbilical 
artery. 

Again,  the  peritoneum,  in  descending  from  the  roof  of  the  cavity  to 
envelop  the  first  part  of  the  rectum,  forms  a  suspensory  fold — the  meso- 
rectum,  which  is  continuous  in  front  with  the  colic  mesentery. 

On  each  side  the  ureter  and  the  vas  deferens  project  narrow  bands  of 
peritoneum,  and  the  right  and  left  vasa  deferentia  where  they  lie  above 
the  bladder  are  connected  by  a  triangular  serous  fold  which  contains 
between  its  layers  the  prostatic  vesicle. 

In  the  mare  there  ai-e  formed  in  an  analogous  manner  the  uterine 
ligaments  described  at  page  303. 

Directiofis. — The  pelvis  should  now  be  either  laid  on  its  side,  or  sus- 
pended in  the  natural  position  and  at  a  convenient  height.  A  side  view 
of  its  contents  is  to  be  exposed  by  the  following  steps  : — With  the  saw 
cut  through  the  shaft  of  the  ilium  close  above  the  cotyloid  cavity.  Make 
another  section  through  the  same  bone  immediately  external  to  the  sacro- 
iliac articulation.  Remove  the  intermediate  piece  of  bone,  at  the  same 
time  separating  the  peritoneum  from  its  inner  aspect.  Find  the  internal 
pudic  artery  in  the  position  shown  in  Plate  1 6.  It  will  be  either  internal 
to  the  sacro-sciatic  ligament  or  in  its  texture.  Trace  it  forwards  and 
backwards.  It  is  accompanied  by  a  satellite  vein,  and  where  the  two 
vessels  pass  above  the  small  sacro-sciatic  foramen  they  are  crossed  out- 
wardly by  the  internal  pudic  nerve.  This  having  been  found  should 
be  followed  iip wards.  Without  injury  to  the  nerve  and  vessels,  the 
sacro-sciatic  ligament  may  then  be  removed,  taking  care  of  the  com- 
pressor coccygis  and  retractor  ani  muscles,  which  lie  internal  to  the  pos- 
terior pai-t  of  the  ligament. 

The  Internal  Pudic  Artery  (Plates  46  and  47)  is  a  branch  of  the 
internal  iliac,  arising  at  the  last  lumbar  vertebra.  Entering  the  pelvis, 
it  descends  obliquely  downwards  and  backwards  across  the  side  of  the 
cavity,  lying  on  the  inner  surfiice  of  the  sacro-sciatic  ligament  or  within 
its  texture  (Plate  16).  At  the  small  sacro-sciatic  foramen  it  passes  with 
an  inward  and  l)ackward  direction,  terminating  in  a  manner  that  varies 
with  the  sex. 

In  the  male  it  turns  round  the  ischial  arch  and  reaches  the  pcrinteum, 
where  it  penetrates  the  urethral  bulb.  Besides  slender  hccmorrhoidal 
and  perineal  branches,  it  gives  off  the  vesico-prostatic  artery. 

The  vesico-prostatic  artery  arises  about  the  neck  of  the  bladder,  and 
supplies  the  prostate,  the  vesicula  seminalis,  +he  posterior  i)art  of  the 
bladder,  and  the  terminal  part  of  the  vas  deferens. 


DISSECTION    OF    THE    PELVIS.  343 

111  the  female  the  internal  pudic  teiininates  in  hsemon'hoidal,  vulval", 
and  bulbous  branches;  and,  instead  of  the  vesico-prostatic,  it  gives  off 
the  vaginal  artery,  Avhich  is  expended  in  the  bladder,  vagina,  and  cervix 
uteri,  anastomosing  with  branches  of  the  uterine  artery. 

The  Umbilical  or  Hypogastric  Artery.  In  the  adult  (Plate  46)  this 
is  a  comparatively  small  vessel  arising  from  the  internal  pudic  near  its 
root.  It  is  pervious  only  in  the  first  few  inches  of  its  course,  giving  off 
a  few  twigs  to  the  bladder,  and  being  then  continued  as  a  solid  cord  at 
the  free  edge  of  the  lateral  ligament  of  the  bladder.  In  the  foetus, 
however,  it  is  of  great  size,  and  carries  the  foetal  blood  to  the  placenta  to 
be  purified. 

The  Internal  Pudic  Vein  runs  in  company  with  the  artery.  It 
receives  branches  corresponding  to  those  of  the  artery,  and  terminates  in 
the  internal  iliac  vein. 

The  Pudic  Nerve  is  derived  from  the  3rd  sacral.  Descending  on  the 
inner  surface  of  the  sacro-sciatic  ligament,  it  crosses  the  internal  pudic 
vessels  superficially  at  the  small  sacro-sciatic  foramen.  Here  it  turns 
slightly  inwards,  and  disappears  beneath  the  ischio-urethral  muscle. 
Having  gained  the  lower  face  of  the  urethra,  it  turns  round  the  ischial 
arch,  and  is  continued  as  the  dorsal  nerve  of  the  penis.  Before  leaving 
the  pelvis,  it  detaches  a  perinceo-anal  branch,  which  gives  twigs  to  the 
muscles  of  the  urethra  and  penis,  and  hsemorrhoidal  branches  that  pass 
upwards  on  the  rectum  to  reach  the  anus,  some  of  them  appearing  to 
terminate  in  the  sphincter.  These  latter  branches  are  crossed  by  de- 
scending branches  from  the  hsemorrhoidal  nerve. 

The  lower  posterior  gluteal  nerve  (Plate  16)  gives  fibres  to  both  the 
trunk  of  the  pudic  nerve  and  its  perinaeo-anal  branch,  and  in  some  cases 
the  latter  derives  the  majority  of  its  fibres  from  this  source. 

In  the  female  the  pudic  nerve  terminates  in  branches  to  the  labia, 
clitoris,  and  constrictor  muscles  of  the  vulva. 

The  Hemorrhoidal  Nerve  is  derived  mainly  from  the  4th  sacral.  It 
descends  on  the  inner  face  of  the  sacro-sciatic  ligament,  and  (for  a  short 
distance)  the  compressor  coccygis  muscle.  It  supplies  a  twig  to  that 
muscle,  and  then  penetrates  it,  or  emerges  between  it  and  the  retractor 
ani.  It  then  divides  into  branches  for  the  retractor  and  sphincter 
muscles  of  the  anus,  and  for  the  skin  of  the  periuEeum. 

The  Retractor  Anl  This  muscle  is  described  with  the  perinseum 
(page  276),  but  it  is  here  exposed  in  the  whole  of  its  extent. 

The  Compressor  Coccygis  (Fig.  48)  arises  from  the  inner  surface  of  the 
sacro-sciatic  ligament,  over  the  superior  ischiatic  spine.  Passing  back- 
wards and  upwards,  it  is  inserted  into  the  last  sacral  and  first  two  coccy- 
geal vertebrae.  By  its  inner  face  it  is  related  to  the  rectum,  except  close 
to  its  insertion,  where  the  edge  of  the  depressor  of  the  tail  intervenes. 

Action. — Acting  with  the  opposite  muscle,  it  forcibly  depresses  the 


344  THE  ANATOMY  OF  THE  HORSE. 

tail,  compressing  it  over  the  perinseum.  Acting  singly,  it  inclines  the 
tail  to  that  side. 

Directions. — The  preceding  two  muscles  should  be  entirely  removed. 
Above  the  rectum  there  will  be  found  the  terminal  portion  of  the  pos- 
terior mesenteric  artery;  and  on  its  side,  the  pelvic  plexus  of  nerves. 

The  Posterior  Mesenteric  Artery  (Plate  46)  is  a  branch  of  the 
abdominal  aorta.  Its  terminal  portion  enters  the  pelvic  cavity  between 
the  layers  of  the  meso-rectum ;  and  passing  backwards  above  the  bowel, 
it  terminates  above  the  anus.  In  its  backward  course  it  detaches 
numerous  branches  to  the  wall  of  the  rectum. 

The  Posterior  Mesenteric  Vein  runs  in  company  with  the  artery. 
Its  initial  portion  is  formed  at  the  posterior  part  of  the  rectum,  by  the 
union  of  hsemorrhoidal  veins,  which  communicate  with  like  branches  of 
the  internal  pudic  vein.  In  the  abdominal  cavity  it  concurs  in  the 
formation  of  the  portal  vein. 

The  Pelvic  Plexus  of  the  sympathetic  nerve.  This  is  an  intricate 
network  of  nerves,  placed  on  the  side  of  the  rectum,  and  distributing 
branches  to  the  pelvic  viscera.  It  receives  in  front  the  offsets  from  the 
posterior  mesenteric  plexus,  and  above  it  is  joined  by  branches  fi'om  the 
inferior  sacral  nerves.  In  both  sexes  it  distributes  branches  to  the 
rectum  and  bladder;  and,  besides,  it  supplies  branches  to  the  prostate, 
vesicula  seminalis,  and  vas  deferens  in  the  male,  and  to  the  vagina  and 
uterus  in  the  female. 

The  Rectum  (Plate  46)  is  the  terminal  segment  of  the  large  intes- 
tines. At  the  entrance  to  the  pelvis  it  is  directly  continuous  with  the 
small  colon,  and  it  terminates  at  the  anus.  Its  initial  portion  resembles 
the  small  colon  in  being  puckered  and  of  comparatively  small  calibre. 
Its  terminal  portion,  on  the  other  hand,  is  dilated  and  sac-like,  forming 
a  large  pouch  in  which  the  faeces  collect. 

In  the  male  it  is  related  inferiorly  to  the  bladder,  vesiculse  seminales, 
vasa  deferentia,  prostate  gland,  and  pelvic  part  of  the  urethra.  In  the 
female  it  is  related  on  the  same  aspect  to  the  vulva,  vagina,  and  uterus. 

Structure. — The  wall  of  the  rectum  resembles  that  of  the  large  intes- 
tine in  general  (page  309),  possessing  serous,  muscular,  submucous,  and 
mucous  layers.  As  ah-eady  seen,  its  peritoneal  investment  is  incom- 
plete, its  terminal  portion  being  destitute  of  peritoneum,  and  connected 
by  loose  areolar  tissue  to  contiguoiis  organs.  In  front  of  the  anus  the 
longitudinal  muscular  fibres  of  the  bowel  form  on  each  side  a  band  that 
passes  upwards  to  be  inserted  into  the  coccygeal  vertebrae.  This,  which 
is  termed  the  snspensorj/  ligament  oj  the  rectum,  forms  a  prominence  at 
the  root  of  the  tail.  At  the  anus  the  last  of  the  circular  muscular  fibres 
form  what  is  termed  the  internal  sphincter.  Developed  in  connec- 
tion with  the  tei-mination  of  the  rectum  are  two  striped  muscles — the 
sphincter  ani  extenius  and  the  retractor  ani.      These  are  described  at 


DISSECTION    OF    THE    PELVIS.  345 

page  276,  In  the  male  the  retractor  muscles  of  the  penis  (page  276)  form 
a  kind  of  sling  for  the  rectum  in  front  of  the  anus ;  and  similar  cords 
of  invohmtary  muscular  tissue  unite  below  the  rectum  at  the  same  point 
in  the  female,  and  terminate  in  the  \ailva. 

The  Ukinary  Bladder  (Plates  46  and  47)  is  the  reserv'oir  for  the 
accumulation  of  the  urine.  The  secretory  action  of  the  kidneys  is 
constant ;  and  the  urine,  passing  along  the  ureter,  accumulates  in  the 
bladder,  to  be  expelled  at  intervals.  As  now  seen  in  its  distended 
condition,  the  bladder  is  not  wholly  contained  within  the  pelvic  cavity, 
but  projects  a  little  beyond  the  pubic  brim.  When  empty,  however, 
it  lies  entirely  within  the  cavity,  resting  on  the  concave  upper  surface 
of  the  pubic  bones.  In  form  the  distended  viscus  is  ovoid.  The  broad 
end,  which  is  free  and  directed  foi-wards,  is  termed  the  fundus ;  the 
narrow  end  has  the  opposite  direction,  and  becomes  continuous  by  a 
constricted  neck  with  the  urethra ;  the  sides  of  the  bladder  are  related 
to  the  pelvic  walls  ;  and  the  upper  surface  is  related  to  the  rectum,  vasa 
deferentia,  and  vesicul£e  seminales  in  the  male,  and  to  the  vagina  and 
uterus  in  the  female.  It  is  maintained  in  position  by  the  peritoneum, 
which  gives  it  only  a  partial  covering,  and  by  its  continuity  with  the 
urethra,  As  already  noticed,  the  peritoneum  in  passing  on  to  it  forms 
the  folds  called  the  middle  and  lateral  ligaments  of  the  organ. 

The  Ureters  (Plates  46  and  47).  Each  tube  having  crossed  the  inlet 
of  the  pelvis,  passes  across  its  lateral  Avail,  sustained  by  a  narrow  band  of 
peritoneum.  Finally,  it  is  reflected  inwards  to  perforate  the  upper  wall 
of  the  bladder,  a  little  in  advance  of  its  neck. 

Directions. — Should  the  subject  be  a  mare,  the  dissector  must  now 
turn  to  page  351  e^  seq.,  where  the  urethra  and  reproductive  organs  of 
the  female  are  described. 

The  Urethra  in  the  male  (Plate  47).  This  is  a  long  tube,  extending 
from  the  neck  of  the  bladder  to  the  free  extremity  of  the  penis.  The 
first  few  inches  of  the  tube  are  contained  within  the  pelvis,  between  the 
rectum  and  the  ischiatic  symphysis  ;  for  the  rest  of  its  extent  it  is  extra- 
pelvic,  and  amalgamated  with  the  penis  except  at  its  termination,  where 
it  projects  as  a  short  tube  from  the  glans  penis.  The  intra-pelvic 
division  of  the  tube  is  divided  into  the  jyrosfatic  and  membranous 
portions ;  .the  extra-pelvic  division  is  called  also  the  spongy  portion. 
The  prostatic  portion  includes  the  first  inch  or  two  of  the  tube  behind 
the  neck  of  the  bladder,  and  it  is  embraced  by  the  prostrate  gland. 
The  membranous  portion  comprises  the  next  two  or  three  inches,  extend- 
ing as  far  as  the  ischial  arch,  where,  at  a  very  acute  angle,  it  becomes 
continuous  with  the  spongy  portion.  It  is  at  this  angle  that  the  point 
of  the  catheter  is  likely  to  be  arrested. 

Muscles.  The  membranous  part  of  the  urethra  has  connected  with 
it  two  muscles.     The  first  of  these,   termed    Wilson's  muscle,   or  the 


346  THE  ANATOMY  OF  THE  HORSK. 

constrictor  vret/ine,  enveloiis  the  tube  behind  the  prostrate  gland,  from 
which,  indeed,  it  is  not  well  defined.  Its  muscular  fibres,  of  a  pale  red 
colour,  comprise  two  sets,  which  extend  across  the  urethra  on  its  upper 
and  lower  faces  respectively,  and  embrace  the  tube  like  an  elliptical 
sphincter.  The  most  posterior  fibres  of  the  muscle  pass  over  Cowper's 
glands.  The  other,  termed  the  ischio-urethral  inuscle,  consists  on  each 
side  of  a  band  whose  fibres  arise  from  the  ischial  arch,  and  pass  to  the 
urethra  beneath  Cowper's  gland,  blending  with  Wilson's  muscle.  Like 
the  preceding,  it  is  composed  of  pale  red  muscular  fibres. 

Action. — These  muscles  are  constrictors  of  the  membranous  urethra, 
and  aid  in  tlie  ejection  of  urine  and  semen. 

The  sponffy  portion  of  the  urethra,  with  its  muscles — tlic  transvei-sus 
perinoei  and  accelerator  urinse — has  been  already  described  as  a  con- 
stituent part  of  the  penis  (page  284). 

The  Prostate  Gland  (Plates  46  and  47)  embraces  the  neck  of  the 
bladder  and  the  initial  part  of  the  urethra.  It  consists  of  a  middle  and 
two  lateral  lobes ;  and  in  structure  it  is  glandular,  with  a  considerable 
admixture  of  striped  muscular  tissue.  Its  glandular  texture  consists  of 
branching  excretory  tubes  and  acini,  both  having  a  columnar  lining. 
Its  ducts,  as  will  be  seen  at  a  later  stage,  perforate  the  urethral  wall,  to 
which  it  is  adherent. 

Cowper's  Glands  (Plates  4G  and  47).  Each  of  these  is  placed  at  the 
side  of  the  membranous  urethra,  just  in  front  of  the  ischial  arch.  They 
are  round,  reddish-yellow,  and  (in  the  stallion)  about  the  size  of  a  hazel 
nut.  They  have  the  racemose  type  of  structure,  and  their  ducts 
perforate  the  adjacent  wall  of  the  urethra. 

The  Vasa  Deferentia  (Plates  46  and  47).  These  are  the  excretory 
ducts  of  the  testicles.  As  already  seen,  each  is  one  of  the  constituents 
of  the  spermatic  cord.  Appearing  at  the  internal  abdominal  ring,  as  a 
tube  about  the  thickness  of  a  goose-quill,  it  is  reflected  backwards  to 
enter  the  pelvis.  Crossing  the  direction  of  the  ureter,  it  places  itself  on 
the  upper  surface  of  the  bladder,  and  expands  to  four  or  five  times  its 
previous  calibre,  forming  what  is  called  the  bulbous  ])07-tioti  of  the  vas 
deferens.  It  then  passes  backwards  beneath  the  vesicula  seminalis ; 
and  contracting  again,  it  terminates  under  the  prostate,  by  uniting  out- 
wardly with  the  neck  of  the  vesicula  to  form  a  short  tube  termed  the 
ejaculatory  duct.  Where  the  vasa  defei'cntia  lie  above  the  bladder,  they 
are  connected  together  by  a  peritoneal  fold  between  whose  layers  there 
is  contained  the  vesicula  prostatica,  or  titertcs  masculinus.  This  is  a  short 
tube  with  a  blind  anterior  end,  and  opening  by  its  posterior  extremity 
into  the  urethriu  It  is  the  homologue  of  the  uterus  and  vagina  of  the 
female. 

The  Vesicul.e  Seminales  (Plates  46  and  47).  These  bodies  are 
placed  between  the  rectum  and  the  posterior  pax-t  of  the  upper  face  of  the 


DISSECTIOX    OF    THE    PELVIS.  347 

bladder.  Each  is  a  small  ovoid  sac,  like  a  miniature  bladder.  The 
anterior  end  of  the  sac  is  rounded  and  free;  the  posterior  end  contracts, 
and  unites  with  the  vas  deferens  to  form  the  ejaculatory  duct.  Only  the 
anterior  half  of  the  vesicula  is  covered  by  peritoneum,  which  in  passing 
between  the  two  bodies  forms  a  small  triangular  serous  fold. 

The  Common  Ejaculatory  Ducts.  Each  of  these  is  a  short  tube 
formed  under  cover  of  the  prostate,  by  the  union,  at  a  very  acute  angle, 
of  the  neck  of  the  vesicula  seminalis  with  the  vas  deferens.  Its  opening 
into  the  roof  of  the  urethra  will  be  presently  exposed. 

Directions. — Carefully  raise  the  fundus  of  the  bladder,  and  cut  its 
peritoneal  and  connective-tissue  adhesions  to  the  sides  and  floor  of  the 
pelvis.  Free,  in  the  same  way,  the  membranous  urethra  at  the  ischial 
arch ;  and  cut  the  crus  penis  and  its  erector  muscle  from  the  tuber 
ischii.  This  will  enable  the  dissector  to  remove  from  the  pelvis  the 
organs  just  described,  while  maintaining  their  mutual  relations.  Lay 
the  bladder  on  a  table  with  its  upper  or  rectal  aspect  downwards,  and 
open  it  by  a  mesial  incision  on  its  lower  face.  Carry  the  incision 
backwards  into  the  urethra,  so  as  to  open  the  Avhole  extent  of  its 
l^rostatic  and  membranous  portions.  Care  must  be  taken  that  the 
incision  in  both  bladder  and  urethra  is  on  the  inferior  face. 

Structure  of  the  Bladder.     This  comprises  four  coats  :— 

1.  The  Serous  or  Peritoneal  Coat.  This,  as  already  seen,  is  an  incom- 
plete investment. 

2.  The  Muscular  Coat  is  composed  of  bundles  of  non-striped  fibres 
arranged  in  all  directions.  Compared  with  its  condition  in  many  other 
animals,  this  coat  is  very  thin  ;  and  its  fascicvili  in  the  distended 
bladder  seem  hardl}-  to  form  a  continuous  layer.  At  the  neck  of  the 
bladder  some  of  the  fibres  have  a  circular  disposition,  forming  the 
sphincter  vesica'. 

3.  The  Submucous  Coat  is  composed  of  vascular  areolar  connective- 
tissue,  and  it  loosely  unites  the  muscular  and  mucous  coats. 

4.  The  Mucous  Coat.  This  forms  a  complete  interaal  lining  for  the 
bladder,  and  in  the  empt}'  viscus  it  is  thrown  into  folds,  or  rugce. 
Observe  the  slit-like  orifices  of  the  ureters,  near  one  another  and  a  little 
anterior  to  the  urethral  orifice  (Fig.  44).  Pass  a  probe  or  bristle  into 
one  of  them,  and  notice  that  the  ureter  perforates  the  wall  very 
obliquely — an  arrangement  which  has  a  valvular  action  in  preventing 
the  regurgitation  of  urine  from  the  distended  bladder.  Between  the 
uretral  and  urethral  orifices  in  the  human  subject  is  a  triangular 
area — the  trigone — over  which  the  mucous  membrane  is  smooth  even  in 
the  contracted  bladder.  In  the  horse,  however,  this  area  is  wrinkled 
like  the  rest  of  the  surface.  The  epithelium  of  the  mucous  membrane 
is  stratified  and  transitional. 

Structure  OF  the  Urethra.     The  spongy  portion  has  been  described 


348 


THE  ANATOMY  OF  THE  HORSE. 


with  the  penis.  The  prostatic  and  luembranous  portions  have  a 
miicmts  lining,  external  to  which  is  a  imiscuhxr  coat  of  non-striped  fibres. 
Observe  the  following  points  in  connection  with  the  interior  of  the 
intra-pelvic  part  of  the  urethra  (Fig.  44).  On 
the  middle  line  of  the  roof  of  the  tube,  close 
behind  the  communication  with  the  bladder,  there 
is  a  mucous  eminence — the  colliadus  seminalis,  or 
verumontanum.  In  the  gelding  this  is  often  small, 
and  sometimes  hardly  recognisable,  but  in  the 
stallion  it  is  sometimes  a  considerable  eminence, 
like  the  tip  of  the  little  finger.  At  each  side  of 
this  projection  is  the  orifice  of  the  ejaculatory 
duct.  These  orifices  in  the  stallion  are  sufficiently 
large  to  permit  of  the  tip  of  the  little  finger 
being  insinuated  into  them.  This  should  be 
remembered,  as  the  point  of  catheter,  if  not 
guided  along  the  floor  of  the  urethra,  might 
easily  pass  into  one  of  them.  At  the  summit  of 
the  colliculus,  and  therefore  on  the  middle  line,  is 
a  very  minute  opening — the  orifice  of  the  utenxs 
masculinus.  Insert  a  fine  bristle  into  it,  and 
giiide  it  on  into  the  tube.  On  the  wall  of  the 
urethra  at  each  side  of  the  colliculus,  observe  an 
irregular  series  of  minute  orifices  which  belong  to 
the  ducts  of  the  prostate  gland.  Behind  these 
on  each  side,  notice  another  series  of  small  open- 
ings with  a  linear  arrangement.  These  are  the 
orifices  of  the  ducts  of  Cowper's  glands.  Insert 
bristles  into  a  few  of  each  set  of  openings,  and 
guide  them  on  into  the  respective  glands.  Close 
to  the  neck  of  the  bladder  the  epithelium  of  the 
urethra  is  of  the  same  character  as  in  the  bladder, 
but  behind  that  point  it  is  simple  and  columnar. 

Structure  of  the  Vesicul^e  Seminales.  The 
walls  of  these  are  composed  of  fibrous,  fibro- 
muscular,  and  mucous  layei-s ;  and  contain  many 
tubular  glands,  which  discharge  their  secretion 
into  the  cavity,  Avhere  it  mixes  with  the  semen. 
The  bulbous  portion  of  each  vas  deferens  has  the 
same  structure. 

Directions. — The  student  must  now  return  to  the  pelvis,  at  the  roof  of 
which  he  is  to  dissect  the  lumbo-sacral  plexus  of  nerves,  and  the 
branches  of  the  internal  iliac  artery  (Plate  48).  Thereafter  he  is  to 
examine  the  pyrifoiinis  and  obturator  internus  muscles. 


Fig.  44. 

Bladder  and  intrapelvic 
PORTION    OF     Urethra 

OPENLD       FROM      BELOW 

{Uyh). 
1.  Viis  deferens;  1'.  Bulb- 
ous part  of  the  s<ame ;  2.  Peri- 
toneal fold  joining  the  vasa 
deferentia  ;  3.  Bliidder  ;  4. 
Vesicula  Beminalis  ;  5.  Ori- 
fices of  ureters  ;  6.  Prostate  ; 
7.  Verumontanum  with  ori- 
fices of  ejaculatory  ducts ;  8. 
Orifice  of  prostatic  vesicle  ; 
9.  Cowper's  gland  ;  10.  Ori- 
fices of  ducts  of  prostate  ;  11. 
Orifices  of  ducts  of  Cowper's 
gland;  12.  Coipus  caver- 
nosum  ;  13.  Corpus  spongi- 
osum with  urethra  in  its 
centre. 


DISSECTION    OP    THE    PELVIS.  349 

The  LuMBO-SACRAL  Plexus  (Plate  48)  is  composed  of  the  anastomos- 
ing nerve  trunks  for  the  supply  of  the  hind  limb.  It  is  formed  by  the 
inferior  primary  branches  of  the  last  three  lumbar  (4th,  5th,  and  6th) 
and  first  two  sacral  nerves,  and  it  receives  also  a  fosciculus  from  the 
corresponding  branch  of  the  3rd  lumbar  nerve.  Each  of  these  roots 
emerges  fi-om  the  intervertebral  foramen  behind  the  vertebra  after 
which  it  is  named ;  thus,  the  root  from  the  6th  lumbar  nerve  emerges 
by  the  intervertebral  foramen  behind  the  6th  lumbar  vertebra,  the  1st 
sacral  root  by  the  first  inferior  sacral  foramen,  and  so  on.  The  branches 
of  the  plexus,  taken  in  order  from  before  to  behind,  are  as  follows  : — 

1.  Iliaco-muscular  Branches,  for  the  psoas  and  iliacus  muscles. 
Two  of  these  are  seen  in  Plate  48,  one  coming  from  the  antei'ior  root 
of  the  plexus,  and  the  other  from  the  anterior  crural  nerve. 

2.  The  Anterior  or  Great  Crural  Nerve.  In  point  of  size,  this 
is  the  second  nerve  of  the  plexus.  It  derives  its  fibres  from  the  first 
two  roots  of  the  plexus  (4th  and  5th  lumbar),  and  from  the  fasciculus 
fiuTiished  by  the  3rd  lumbar  nerve. 

3.  The  Obturator  Nerve  derives  its  fibres  from  the  4th  and  5th 
lumbar  roots  of  the  plexus.  It  descends  in  company  with  the  obturator 
vessels,  resting  on  the  pelvic  surface  of  the  ilium.  Under  cover  of  the 
obturator  internus  muscle,  it  passes  through  the  obturator  foramen  and 
reaches  the  thigh. 

The  5th  lumbar  root,  having  given  a  branch  to  aid  in  the  formation 
of  the  anterior  crural,  and  another  to  the  obturator  nerve,  is  continued 
obliquely  backwards  between  the  internal  iliac  artery  and  the  spine,  to 
join  a  broad  nervous  fasciculus  to  which  the  remaining  roots  of  the 
plexus  (6th  lumbar  and  first  two  sacral)  contribute  the  whole  of  their 
fibres.  The  remaining  branches  of  the  plexus  are  divisions  of  this 
fasciculus. 

4.  The  Anterior  Gluteal  Nerves.  Three  or  four  in  number,  these 
leave  the  pelvis  and  reach  the  hip  by  passing  through  the  forepart  of 
the  great  sciatic  opening,  with  the  gluteal  vessels. 

5.  The  Great  Sciatic  Nerve,  the  lai-gest  in  the  body,  passes  out  into 
the  hip  through  the  great  sciatic  foramen,  behind  the  preceding. 

6.  The  Posterior  Gluteal  Nerves,  distinguished  as  superior  and 
inferior,  pass  out  behind  the  great  sciatic. 

The  3rd  Sacral  Nerve.  The  inferior  primary  branch  of  this  nerve 
is  continued  as  the  internal  pudic  nerve,  after  giving  a  bundle  of  fibres 
to  aid  in  the  formation  of  the  hsemorrhoidal  nerve. 

The  4th  Sacral  Nerve  receives  the  before-mentioned  branch  from 
the  3rd  nerve,  and  is  continued  as  the  hsemorrhoidal  nerve. 

The  5th  Sacral  Nerve  gives  a  backward  twig  to  the  1st  coccygeal 
nerve,  and  is  then  expended  in  the  skin  of  the  anus  and  root  of  the  tail. 

As  in  other  regions  of  the  spine,  each  of  the  inferior  primary  branches 


350  THE  ANATOMY  OF  TUE  HORSE. 

just  considered  communicates  with  the  contiguous  ganglion  of  the  sym- 
pathetic cord,  by  one  or  more  branches  detached  at  the  intervertebral 
foramen;  and  the  sacral  nerves  send  each  a  filament  to  the  pelvic  plexus. 

The  Sympathetic  Gaxgliated  Cord  in  the  sacral  region.  This  is  the 
direct  backward  continuation  of  the  lumbar  cord.  It  is  placed  on  the 
inferior  surface  of  the  saciiim,  internal  to  the  inferior  sacral  foramina, 
the  latei'al  sacral  artery  interv-ening  between  it  and  the  inferior  primary 
branches  of  the  sacral  nerves  at  their  points  of  emergence.  It  possesses 
a  ganglion  opposite  each  of  the  first  three  sacral  foramina;  and,  as  before 
said,  it  communicates  by  filaments  passing  between  these  ganglia  and 
the  coiTCsponding  spinal  nerves.  The  emergent  branches  of  these 
ganglia  are  very  slender,  and  pass  to  the  cellular  tissue  beneath  the 
saci-um,  or  to  the  contiguous  blood-vessels.  The  cord  terminates  at  the 
last  ganglion,  either  abniptly,  or  by  a  filament  passing  on  to  the  middle 
coccygeal  artery. 

The  Internal  Iliac  Artery  (Plate  48).  This  is  one  of  the  terminal 
branches  of  the  posterior  aorta.  Beginning  at  the  intei'vertebral  disc 
between  the  5th  and  6th  lumbar  vertebrte,  it  passes  downwards  and 
backwards  across  the  articulation  between  the  last  lumbar  transverse 
process  and  the  sacnim,  and  then  across  the  sacro-iliac  articulation;  and 
at  the  upper  part  of  the  ilio-pectineal  line,  a  little  above  the  eminence  of 
the  same  name,  it  divides  into  the  iliaco-muscular  and  obturator  ai-teries. 
The  vessel  is  covered  by  the  peritoneum,  and  in  the  first  inch  or  two  of 
its  course  it  is  separated  from  the  external  iliac  artery  by  the  common 
iliac  vein.  The  collateral  branches  of  the  internal  iliac,  taken  in  the 
order  of  their  point  of  detachment,  are  as  follows: — 

1.  The  second  last  of  the  series  of  lumbar  arteries  arises  from  the  in- 
ternal iliac  at  its  root.  It  behaves  like  the  lumbar  bi-anches  of  the  aorta. 
Its  upper  division,  much  the  larger  of  the  two,  passes  iipwards  through 
the  intervertebral  foramen  between  the  5th  and  6th  lumbar  vertebrre. 

2.  The  Internal  Pudic  Artery.  This  is  a  considerable  vessel  having 
its  origin  at  the  last  lumbar  vertebra.  Entering  the  pelvis,  it  descends 
at  the  ischiatic  edge  of  the  ilium,  and  then  passes  backwards  in  the 
texture  of  the  sacro-sciatic  ligament,  or  on  its  inner  foce. 

3.  The  Lateral  Sacral  Artery  leaves  the  parent  trunk  at  the  sacro- 
lumbar  articulation,  and  passes  backwards  on  the  lower  ftice  of  the 
sacrum,  beneath  or  at  the  inner  side  of  the  inferior  sacral  foramina. 
A  little  behind  the  middle  of  the  sacrum  it  divides  into  the  ischiatic  and 
lateral  coccygeal  arteries.  The  former,  much  the  larger  of  the  two,  passes 
out  through  the  edge  of  the  sacro-sciatic  ligament  to  reach  the  hip 
(Plate  16);  the  latter  continues  the  direction  of  the  lateral  sacral  aileiy 
to  the  tail.  The  inferior  division  of  the  3rd  sacral  nerve  appears  in 
the  angle  of  separation  between  these  two  arteries.  The  collateral 
branches  of  the  lateral  sacral  artery  are  : — (1)    Branches  entering  the 


DISSECTION    OF    THE    PELVIS.  351 

intervertebral  foramen  between  the  last  lumbar  vertebra  and  the  sacrnm 
(Last  lumbar  artery),  and  the  first  two  or  three  inferior  sacral  foramina. 
Each  of  these  enters  the  spinal  canal,  furnishes  there  a  spinal  branch, 
and  then  emerges  by  the  corresponding  superior  foramen,  and  is  dis- 
tributed to  the  overlying  muscles  and  skin.  (2)  The  middle  coccygeal 
artery  is  an  unpaired  vessel,  variable  as  to  its  origin,  but  generally, 
as  in  Plate  48,  furnished  by  the  right  lateral  sacral  artery.  It  passes 
inwards  to  the  middle  line,  and  is  continued  backwards  to  the  tail. 

4.  The  Ilio-lumbar  Artery.  This  artery  is  in  series  with  the  lumbar 
arteries,  representing,  as  it  were,  the  abdominal  or  inferior  branch  of  the 
last  lumbar  artery.  Arising  from  the  outer  side  of  the  parent  trunk,  it 
passes  outwards  across  the  sacro-iliac  joint,  giving  branches  to  the  iliacus 
and  psoas  magnus  muscles.  Its  terminal  twigs  may  reach  the  gluteus 
maximus  or  the  tensor  vaginae  femoris. 

5.  The  Gluteal  Artery,  a  large  vessel,  arises  at  the  edge  of  the 
sacrum,  and  passes  out  into  the  hip  by  the  great  sacro-sciatic  foramen, 
dividing  into  a  number  of  branches  as  it  escapes  (Plate  16). 

The  Iliaco-femoral  Artery,  one  of  the  terminal  branches  of  the 
internal  iliac,  passes  do^\^l wards  and  outwards  bet^^'een  the  shaft  of  the 
ilium  and  the  iliacus  muscles,  to  reach  the  outer  aspect  of  the  thigh.  It 
supplies  the  nutrient  artery  of  the  ilium. 

The  Obturator  Artery,  the  other  terminal  branch  of  the  internal 
iliac,  passes  dowaiwards  and  backwards  on  the  pelvic  surface  of  the  ilium, 
at  the  anterior  edge  of  the  pyriformis  muscle.  Under  cover  of  the 
obturator  intemus  muscle,  it  passes  through  the  obturator  foramen  and 
reaches  the  thigh.  It  is  accompanied  by  a  satellite  vein,  and  by  the 
obturator  nerve,  which  is  pkced  anterior  to  the  vessels.  The  tendon  of 
the  psoas  parvus  muscle  is  inserted  in  the  angle  of  separation  between 
this  and  the  preceding  artery. 

The  Internal  Iliac  Vein  collects  the  blood  from  the  satellite  veins 
of  the  foregoing  arteries.  It  unites  with  the  external  iliac  vein,  forming 
the  common  iliac  vein. 

The  Obturator  Internus  and  the  Pyriformis.  For  a  description  of 
these  muscles  turn  to  page  68. 

reproductive  organs  in  the  female. 

Comprised  under  this  heading  there  are  :  the  ovaries,  the  Fallopian 
tubes,  the  uterus,  the  vagina,  and  the  vulva.  The  ovaries,  the  Fallopian 
tubes,  and  the  uterus  (in  part)  are  abdominal  organs,  and  their  mode  of 
suspension  in  that  cavity  has  already  been  noticed.  Their  moi-e  com- 
plete examination  can  now  be  undertaken  along  with  the  dissection 
of  the  purely  pelvic  parts  of  the  same  apparatus,  and  at  the  same  time 
it  is  convenient  to  examine  the  female  urethra. 

The  Ovaries,  as  already  seen,  are  situated  in  the  lumbar  region  of  the 


352  THE  ANATOMY  OF  THE  HORSE. 

abdominal  cavity  (see  page  303).  Each  ovary  is  about  half  the  size  of 
the  testicle — the  corresponding  organ  of  the  male.  In  form  it  is  ovoid, 
with  a  distinct  depression  on  its  upper  surfixce — the  hilus.  At  the  hilus 
the  nerves  and  vessels  of  the  organ  enter  from  the  broad  ligament  of  the 
uterus,  and  in  its  neighbourhood  the  expanded  end  of  the  Fallopian  tube 
is  attached  by  one  of  its  fimbrice  to  the  surftxce  of  the  ovary.  From  the 
posterior  extremity  of  the  ovary  a  cord  of  non-striped  muscular  tissue — 
the  ligament  of  the  ovary — passes  to  the  uterine  coniiu  The  lateral 
surfaces,  the  inferior  border,  and  the  anterior  end  of  the  ovary  are 
rounded  and  free. 

Structure  of  the  Ovarv.  This  comprises  (1)  an  epithelial  covering 
on  the  surface  of  the  organ,  (2)  a  fibrous  framework,  or  stroma,  and 
(3)  Graafian  follicles. 

1.  The  Germinal  Epithelium. — This  is  a  single  layer  of  short  columnar 
cells  with  granular  contents.  In  veterinary  text-books  the  surface  of 
the  ovary  is  desci'ibed  as  having  a  serous  covering  derived  from  the 
broad  ligaments.  The  cells  of  this  surface  covering,  however,  are  in 
marked  contrast  to  the  cells  of  the  broad  ligament,  which  have  the 
ordinary  flattened  and  transparent  endothelial  characters.  The  term 
germinal  is  applied  to  this  layer  because  the  ova,  or  gei-m-cells,  are 
separated  from  it  in  the  fojtal  ovary. 

2.  T\\e  Stroma  is  composed  of  fibrous  connective-tissue  with  some  bundles 
of  non-striped  muscular  tissue.  The  blood  vessels  of  the  ovary  ramify 
in  it,  and  it  surrounds  the  Graafian  follicles.  Around  the  hilus  it  is 
most  vascular  and  open  in  texture,  and  this  portion  of  the  stroma  is 
sometimes  termed  the  zona  vasculosa  or  the  medullary  substance,  in  contra- 
distinction to  the  peripheral  cortical  substance.  A  layer  of  condensed 
stroma  without  any  Graafian  follicles  lies  beneath  the  surface  epithelium, 
and  is  sometimes  termed  the  tunica  albuginea  of  the  ovary. 

3.  The  Graafian  Follicles,  or  Ovisacs. — These  are  vesicular  bodies  for 
the  matui'ation  and  extrusion  of  the  ova.  A  large-sized  follicle  possesses 
the  following  parts  : — 

a.  The  wall  of  the  follicle,  composed  of  an  inner  delicate  tunica 
propria,  and  an  outer  layer — the  tunica  fibrosa — derived  from  the 
surrounding  stroma. 

6.  The  Menibrana  Gramdosa. — This  forms  an  epithelial  lining  to  the 
wall  of  the  follicle,  and  consists  of  several  layers  of  cells.  At  one  point 
these  epithelial  cells  are  heaped  up  to  form  the  cumulus  or  discus 
ptroligerus,  the  cells  of  which  surround  the  ovum. 

c.  The  Liquor  Folliculi. — This  is  a  fluid  which  fills  up  the  remainder  of 
the  cavity  of  the  follicle. 

The  Ovum  is  a  typical  animal  cell.  It  consists  of  an  outer  envelope — the 
zonaj)ellucida;  protoplasmic  cell-contents — the  vitellus  or  yelk;  a  nucleus — 
the  germinalvesicle;  and,  within  the  nucleus,  a  nucleolus — i\\Q germinal  spot. 


DISSECTION    OF   THE   PELV] 


353 


The  Graafian  follicles  vary  greatly  in  size.  The  smallest  are  imbedded 
in  the  cortical  part  of  the  ovary.  These  are  of  microscopic  size,  and 
differ  from  the  larger  follicles  in  having  only  a  single  layer  of  cells  in 
the  membrana  granulosa,  and  in  having  no  liquor  folliculi.  Follicles  of 
intermediate  size  are  placed  more  deeply  in  the  ovary,  and  differ  from  the 
largest  chiefly  in  the  small  amomit  of  liquor  that  they  contain.  These 
differences  of  size  represent  different  stages  of  development  of  the 
follicles,  the  largest  being  the  most  mature.  When  mature,  a  follicle 
occupies  a  considerable  space  in  the  substance  of  the  ovary  in  the  neigh- 
bourhood of  the  hilus.     Finally  it  bursts  through  the  surface  of  the 


Fig.  45. 

Sl-ctiox  of  Cat's  Ovary,  magnified  (from  Schron). 

1.  Outer  covering  of  the  ovary  ;  2.  Fibrous  stroma  ;  3.  Superficial  layer  of  fibro-nuclear  substance; 
3'.  Deeper  parts  of  the  same  ;  4.  Blood-vessels  ;  5.  Ovi-sacs  forming  a  layer  near  the  surface  ;  6.  One 
or  two  of  the  ovi-sacs  sinking  deeper  and  beginning  to  enlarge  ;  7.  One  of  the  ovi-sacs  farther 
developed,  now  enclosed  by  a  prolongation  of  the  fibrous  stroma,  and  consisting  of  a  small  Graafian 
follicle,  within  which  is  situated  the  ovum  covered  by  the  cells  of  the  discus  proligerus  ;  8.  A  follicle 
farther  advanced ;  8'.  Another  which  is  irregfularly  compressed  ;  9.  the  greater  part  of  the  largest 
follicle,  in  which  the  following  parts  are  seen  ;  a.  Cells  of  the  tunica  granulosa  lining  the  follicle  ; 
6.  The  reflected  portion  named  discus  proligerus  ;  c.  Vitellus  or  yelk  part  of  the  ovum,  surrounded 
by  the  zona  pellucida  ;  d.  germinal  vessicle  ;  e.  Germinal  spot. 

ovary,  and  the  ovum,  along  with  the  liquor  folliculi  and  part  of  the 
membrana  granulosa,  escapes  and  is  caught  by  the  expanded  extremity 
of  the  Fallopian  tube.  The  follicle  then  collapses,  while  it  becomes  in 
part  filled  with  blood  from  the  vessels  opened  by  the  rupture  of  its  wall. 
The   rupture   then  heals,   and  the  follicle  becomes  converted    into   a 

2  A 


354 


THE  ANATOMY  OF  THE  HOUSE. 


yellowish  body — the  corjms  lutenm.  In  the  early  stage  of  a  corpus 
luteum  the  cells  of  the  niembrana  granulosa  proliferate,  while  capil- 
laries extend  into  it  from  the  wall  of  the  follicle.  Later  on  the  blood- 
clot  in  the  centre  becomes  decolorised,  and  the  granulosa  cells  become 
fotty ;  and  finally  the  corpus  luteum  shrinks  and  disappears. 

The  ParovariUiM,  or  the  Organ  of  Kosenmuller.  This  is  a  minute 
body  situated  in  tlie  broad  ligament,  between  the  ovary  and  the 
Fallopian  tube.  It  consists  of  a  number  of  short  convoluted  tubules 
opening  into  a  longitudinal  tube,  the  latter  representing  the  caiud  of 
Gcertner  in  the  cow.  The  parovarium  is  the  homologue  of  the  epididymis 
of  the  male. 

The  Fallopian  Tubes,  or  Oviducts.  The  Fallopian  tube  is  the  duct 
for  the  conveyance  of  the  ova  from  the  ovary  to  the  uterus.     In  its 


Fk;.  40. 

Right  Ovary  and  Fallopian  Ti'bk. 

1.  FaUopian  tube  ;  2.  Abdominal  oijening  (fimbriated  extremity)  of  tlie  same  ;  3.  A  probe  intro- 
duced into  the  uterine  opening  of  the  tube  ;  4.  Ovary  ;  5.  Ligament  of  the  ovary  ;  6.  Broad  liga- 
ment of  the  uterus  ;  7.  Tip  of  uterine  cornu  laid  open. 

course  between  these  two  organs  the  tube  passes  in  a  flexuous  manner 
at  the  anterior  border  of  the  broad  ligament.  The  ovarian  extremity 
of  the  tube  opens  on  the  surface  of  an  expansion  whose  rim  is  cut  into 
a  few  short  fringe -like  processes — the  fiinbrioi.  Inwardly  the  rim 
of  this  expansion  is  fixed  to  the  surfiice  of  the  ovary  near  the  hilus. 
The  upper  surfiice  of  the  expansion  is  covered  by  a  mucous  membrane 
with  delicate  rugse  that  converge  from  its  rim  to  its  centre,  where  it 
shows  the  orifice  of  the  tube — the  ostium  ahdominale.     The  under  surfixce 


DISSECTION    OF    THE    PELVIS.  355 

of  the  expansion  is  smooth  and  covered  by  peritoneum.  The  uterine 
extremity  of  the  tube  opens  into  the  extremity  of  the  uterine  liorn  by  a 
minute  orifice — the  ostium  uterinum. 

Although  the  Fallopian  tube  bears  to  the  ovary  the  relationship  of  an 
excretory  duct,  in  that  it  conveys  away  the  ova,  it  differs  from  all  other 
excretory  ducts  in  not  having  its  lumen  closely  continuous  with  the 
interior  of  the  gland  whose  secretion  it  conveys.  Moreover,  this  discon- 
tinuity between  the  Fallopian  tube  and  the  ovary  establishes  an  indirect 
communication  between  the  sac  of  the  peritoneum  and  the  surftice  of  the 
body,  and  brings  about  the  single  exception  to  the  rule  that  serous 
membranes  form  perfectly  close  sacs. 

Structure  of  the  Tube.  The  wall  of  the  oviduct  comprises  the  follow- 
ing layers,  enumerated  from  without  inwards,  viz.,  (1)  an  outer  serous  coat, 
derived  from  the  broad  ligament ;  (2)  a  coat  of  non-striped  muscular  tissue, 
arranged  as  an  outer  longitudinal  and  an  inner  circular  set  of  fibres;  (3) 
a  submucoiis  coat  of  vascular  connective-tissue ;  (4)  a  mticous  coat,  having 
a  ciliated  columnar  epithelium. 

The  lumen  of  the  tube  is  narrowest  at  its  uterine  extremity  and  widest 
at  the  ovary. 

The  Uterus,  or  womb,  is  the  organ  that  receives  the  ovum,  retains  it 
during  its  development  (provided  it  has  been  fertilised),  and,  finally, 
expels  it  at  the  expiration  of  the  full  term  of  pregnancy.  In  situation 
the  oi'gan  is  partly  abdominal,  and  partly  pelvic,  and  its  mode  of 
suspension  by  the  broad  ligaments  has  already  been  observed  in 
connection  with  the  peritoneum  (page  303). 

The  organ  is  single  in  its  posterior  portion,  and  bifid  in  front. 

The  anterior  bifurcations  of  the  organ  are  termed  its  cornua  or  horns. 
At  its  anterior  extremity  each  horn  is  pointed,  and  receives  the  uterine 
opening  of  the  Fallopian  tube.  From  this  point  the  calibre  of  the  horn 
gradually  increases  to  its  posterior  end,  where  it  opens  into  the  body  of 
the  organ.  Each  horn  shows  a  concave  upper  border  at  which  the  broad 
ligament  reaches  it,  while  its  lower  border  is  convex  and  free.  The 
cornua  are  entirely  abdominal  in  position,  and  are  related  to  the 
intestines. 

The  posterior  single  portion  of  the  uterus  comprises  the  body,  and  the 
neck,  or  cervix;  but  this  division  is  not  apparent  on  the  exterior. 

The  body,  placed  in  front,  presents  two  faces,  two  borders,  and  two 
extremities.  The  upper  face  is  slightly  flattened  and  related  to  the 
rectum ;  the  lower  fiice,  also  flattened,  is  related  to  the  intestines  in 
front,  and  to  the  bladder  behind ;  the  borders,  right  and  left,  show  the 
insertions  of  the  broad  ligaments;  the  anterior  extremity,  or  fitndus,  is 
the  widest  part  of  the  body,  and  it  is  joined  at  each  angle  by  the  cornu ; 
the  posterior  extremity  is  continuous  with  the  cervix.  The  body  of 
the  uterus  is  partly  abdominal  and  partly  pelvic  in  situation. 


856 


THE    ANATOMY    OF    THE    HOH.SE. 


The  ceriix  is  the  extreme  posterior  part  of  uterus.  It  is  directly 
continuous  with  the  body  in  front ;  and  its  posterior  extremity,  as  will 
be  seen  when  the  organ  is  laid  open,  projects  into  the  anterior 
extremity  of  the  vagina. 

The  Vagina  is  a  tubular 
organ  which  connects  the  uterus 
and  the  vulva.  It  is  lodged 
entirely  within  the  pelvis,  being 
related  to  the  rectum  above,  to 
the  bladder  and  urethra  below, 
and  to  the  iiretei-s  and  pelvic 
walls  laterally.  Its  mode  of 
comiection  with  the  two  cavities 
that  it  connects  will  be  ex- 
amined later  on.  Its  average 
length  is  about  nine  or  ten 
inches. 

The  Vulva  is  the  passage 
that  continues  the  vagina  back- 
wards, and  opens  on  the  surface 
of  tlie  body  beneath  the  anus. 

The  tube  of  the  vulva  is 
about  five  inches  in  length.  It 
is  united  by  cellular  tissue  to 
the  rectum  above,  and  to  the 
pelvic  floor  below,  while  on 
each  side  it  is  related  to  the 
retractor  ani  muscle.  Below 
and  laterally  it  is  covered  by  a 
layer  of  striped  muscular  tissue 
— the  antei-ior  constrictor  of  the 
vulva.  The  fibres  of  this  muscle  after  embracing  the  tube  of  the  ■\'xilva 
are  lost  on  the  sides  of  the  rectum. 

The  external  opening  of  the  ^'^^lva  has  the  form  of  a  vei-tical  slit,  and 
it  is  bounded  at  the  sides  by  the  labia,  which  meet  above  and  below  to 
form  the  cornmissio'es.  The  superior  commissure  is  aciite,  and  separated 
from  the  anus  by  a  narrow  interval.  The  inferior  commissure  is  rounded, 
and  immediately  within  it  the  clitoris  is  lodged.  The  labia  are  covered 
externally  by  skin,  which  is  thin,  almost  destitute  of  hairs,  and  generally 
black-pigmented ;  inwardly  they  are  lined  by  mucous  membrane  ;  and 
at  their  sharp  edges  these  cutaneo\is  and  mucous  coverings  meet.  If  the 
cutaneous  covering  of  the  labia  be  removed,  the  posteiior  constrictor  of 
the  vulva  will  be  exposed.  This  is  a  red  muscle  corresponding  to  the 
compressor  bulbi  of  human  anatomy.     Its  fibres  are  elliptically  disposed 


~>m^^^ 

m" 

JL»VJP^^^ 

m 

rrt        V 

..  771 

Fig 

.  47. 

Generative  Organs   of  the  Mare,  viewed 

ABOVE. 

1,  1.  Ovaries;  2,  2.  Fallopian  tubes;  3.  FimTjiiuted 
extremity  of  the  tube,  outer  face  ;  4.  The  same,  inner 
face,  showing  the  abdominal  orifice  ;  5.  Ligament  of 
the  ovary;  (>.  Right  cornu,  intact;  7.  Left  cornu, 
laid  open  ;  S.  Body  of  the  uterus  ;  i).  Broad  ligament  ; 
10.  Os  uteri  (externum);  11.  Interior  of  the  vagina; 
12.  Meatus  urinarius,  with  its  valve  13  ;  14.  Mucous 
fold,  a  vestige  of  tlie  hymen  ;  15.  Interior  of  the 
vulva  ;  16.  Clitoris  ;  17,  17.  Labia  of  the  vulva  ;  IS. 
Inferior  commissure  of  the  vulva. 


mSSEOTION    OF    THE    PELVIS.  357 

around  the  extremity  of  the  vulva,  being  confounded  with  the  sphincter 
ani  above,  while  inferiorly  some  of  the  fibres  are  attached  to  the  base 
of  the  clitoris,  and  others  are  attached  to  the  inner  surface  of 
the  skin  below  the  inferior  commissure.  When  the  muscle  contracts,  it 
constricts  the  orifice  of  the  vulva.  Its  lower  fibres  may  frequently  be 
observed  to  contract  after  micturition,  depressing  the  inferior  commissure 
and  exposing  the  clitoris,  which  is  simultaneously  erected. 

The  Clitoris.  This  small  erectile  body  is  the  homologue  of  the  male 
penis  minus  the  urethra.  It  is  lodged  within  the  inferior  commissiire  of 
the  vulva,  and  presents  a  base,  or  attached  extremity,  a  body,  and  a  free 
extremity.  The  base  is  bifid,  and  attached  to  the  ischial  arch  by  the 
branches,  or  crura,  each  crus  being  covered  by  a  rudimentary  erector 
clitoridis  muscle — the  homologue  of  the  erector  penis.  The  body  of  the 
clitoris,  which  is  from  two  to  three  inches  in  length,  projects  backwards 
and  upwards,  and  is  composed  of  right  and  left  halves  like  the  corpora 
cavernosa  of  the  penis.  The  free  extremity  is  formed  by  a  nidimentary 
glans,  which  is  provided  with  a  mucous  cap  analogous  to  the  prepuce. 
The  clitoris  is  composed  of  erectile  tissue  resembling  that  of  the 
penis. 

The  Vestibular  Bulb.  This  will  be  exposed  by  the  removal  of  the 
posterior  constrictor  muscle.  It  is  an  erectile  body  composed  of  right 
and  left  halves,  each  of  which  is  placed  at  the  side  of  the  vulvar  cavity 
(the  vestibule),  between  the  posterior  constrictor  and  the  mucous 
membrane.  Inferiorly  the  two  halves  of  the  organ  are  in  communica- 
tion with  one  another,  and  with  the  erectile  tiss\ie  of  the  clitoris, 
and  superiorly  each  terminates  at  the  side  of  the  vulva  by  a  rounded 
end.  The  bulb  is  the  homologue  of  the  corpus  spongiosum  of  the 
penis. 

Directions. — The  pelvic  viscera  must  now  be  removed  to  allow  an 
examination  of  the  structure  and  interior  of  the  organs  just  considered. 
Tliis  is  to  be  effected  by  cutting  the  meso-rectum  and  the  peritoneal 
ligaments  of  the  bladder,  carrying  the  knife  above  the  anus  and  below 
tlie  inferior  commissure  of  the  vulva,  and  destroying  the  vascular 
and  connective-tissue  attachments  of  the  various  organs  to  the  pelvic 
walls.  The  entire  generative  apparatus  will  thus  be  removed  along 
with  the  urinary  bladder  and  the  rectum.  The  latter  organ  should  be 
dissected  from  the  vagina  and  vulva  (for  its  structure  see  page  344), 
and  the  other  viscera  examined  seriatim.  The  canal  of  the  vulva  and 
vagina  is  to  be  exposed  by  a  mesial  incision  on  the  upper  wall  of  these 
organs. 

The  Canal  of  the  Vulva.  This,  as  already  stated,  is  a  tubular 
passage  about  five  inches  in  length.  When  removed  from  the  body 
and  inflated,  it  assumes  a  large  calibre,  but  ordinarily  its  walls  are 
in  contact.     Tracing  the  canal   in  an   order    inverse   to  that  followed 


358  THE  ANATOMY  OF  THE  HORSE. 

in  the  previous  description  of  parts,  it  may  be  said  to  begin  on  the 
snrfiice  of  the  body  at  the  vertical  slit  already  described,  and  to 
terminate  in  front  by  joining  the  tnbe  of  the  vagina.  In  the 
adult  animal  there  is  little  to  mark  the  line  of  separation  between  the 
two  passages,  but  in  the  young  animal  a  membranous  septiuu — the 
hymen — stretches  between  the  two.  This  is  occasionally  seen  also  in 
the  adult  mare,  and  more  frequently  a  few  warty  projections — the 
canincnloi  myrtiformes — which  are  the  shrunken  remains  of  the  hymen, 
stud  the  line  of  junction  ;  but  very  often  the  canal  of  the  vulva  passes 
without  interniptiou  into  that  of  the  vagina.  The  vulva  is  lined  by  a 
mucous  membrane  of  a  rosy,  vascular  tint.  It  possesses  numerous 
mucous  follicles ;  and  its  free  surface  is  formed  by  a  stratified  squamous 
epithelium,  which,  towards  tlie  external  opening,  is  often  pigmented  in 
spots. 

The  Meatus  Urixarius.  The  urethra  opens  on  the  middle  line  of 
the  floor  of  the  vulva  immediately  behind  its  point  of  continuity  with 
the  vagina.  The  opening  is  surmounted  by  a  large  mucous  fold — 
the  valve  of  the  meatus  urinarius.  This  valve  has  its  free  edge 
directed  backwards,  and  it  serves  to  direct  the  flow  of  urine  towards 
the  exterior.  Its  presence  must  be  remembered  in  passing  the 
female  catheter,  the  point  of  which  should  be  made  to  press  on  tlie  floor 
of  the  vulva  as  it  is  directed  onwards.  The  meatus  is  of  large  size 
when  compared  with  the  same  orifice  in  the  male,  since  it  readily  admits 
two  fingers. 

Directions. — Reverse  the  natural  position  of  parts,  laying  the  uterus, 
vagina,  and  vulva  with  their  upper  surfaces  downwards,  and  open  the 
bladder  by  a  mesial  incision  on  its  lower  {in  the  natiiral  position)  face. 
For  an  account  of  the  structure  of  the  bladder  turn  to  page  347. 

The  Urethra  of  the  female  is  very  much  shorter,  but  considei-ably 
wider,  than  the  corresponding  tube  of  the  male.  Beginning  as  a  fiuniel- 
likc  prolongation  of  the  neck  of  the  bladder,  it  passes  backwards  on  the 
middle  line  of  the  lower  face  of  the  vagina,  in  whose  wall  it  is  partially 
imbedded ;  and  after  a  coui-se  of  two  or  three  inches  it  perforates  the 
lower  wall  of  the  vulva,  and  opens  by  the  meatus  already  described. 
The  calibre  of  the  tube  is  in  correspondence  with  the  large  size  of  the 
meatus ;  and  with  slight  stretching  it  will  accommodate  three  fingers. 
The  wall  of  the  urethra  is  composed  of  connective-tissue,  and  non- 
striped  muscular  fibres  circularly  disposed ;  and  it  is  lined  internally 
V)v  a  longitudinally  folded  mucous  membrane  with  a  stratified  squamous 
epithelium. 

Structure  and  Interior  of  the  Vagina.  The  tube  of  the  vagina 
is  about  nine  or  ten  inches  in  length.  Posteriorly  it  joins  the  vulva, 
and  anteriorly  it  embraces  the  cervix  uteri.  The  connection  between 
the  cavities  of  the  vagina  and  uterus  is,  thus,  not  by  simple  continuity, 


BISSBCTIOX    OF    THE    PELVIS.  359 

but  the  vaginal  wall  is  can-ied  forwards,  so  as  to  cause  the  os  uteri  to 
project  freely  into  the  forepart  of  the  vaginal  canal. 

The  wall  of  the  vagina  comprises  the  following  layers  : — 

1.  A  Serotis  Coat. — This  is  only  a  partial  covering,  the  posterior  part 
of  the  organ  being  Avithout  a  peritoneal  investment.  In  the  hinder  part 
of  the  tube  the  place  of  the  peritoneum  is  taken  by  connective-tissue 
uniting  it  to  surrounding  parts.  This  connective-tissue  is  loose  and 
areolar  towards  the  rectum  ;  but  between  the  vagina  and  the  bladder  it 
is  closer,  and  forms  a  more  intimate  bond  between  the  two  organs. 

2.  A  Muscular  Coat. — This  is  composed  of  non-striped  muscular 
tissue,  continuous  in  front  with  the  muscular  coat  of  the  uterus. 
Posteriorly  the  muscular  tissue  is  reddish  in  tint,  and  passes  into  the 
anterior  constrictor  of  the  vulva.  The  fibres  are  arranged  both  longi- 
tudinally and  circularly. 

3.  A  Mucotis  Coat.  This  lines  the  tube  inwardly,  and  it  is  longitudi- 
nally folded.  It  possesses  numei'ous  mucous  glands,  and  its  epithelium 
is  stratified  and  squamous.  It  is  of  a  pinkish,  vascular  tint,  like  the 
mucous  lining  of  the  vulva. 

Directions. — Lay  open  one  of  the  horns  of  the  uterus  in  its  whole 
extent,  and  carry  the  incision  along  the  body  and  cervix  to  the  os. 

Structure  and  Interior  of  the  Uterus.  The  interior  of  the  uterus 
comprises  the  cavities  of  the  body  and  cervix,  and  those  of  the  horns. 

The  Cavity  of  the  Cervix  begins  posteriorly  at  the  orifice  of  the  tap- 
like projection  already  noticed  at  the  forepart  of  the  vaginal  canal. 
This  orifice  is  termed  the  os  uteri  externum,  or,  shortly,  the  os  uteri. 
Ordinarily  the  orifice  is  closed,  and  forms  a  circular  depression  from 
which  the  folds  of  mucous  membrane  radiate  outwards,  and  curve  round 
the  circular  lip  of  the  os,  to  be  carried  to  the  vaginal  wall. 

In  front  the  canal  of  the  cervix  passes  gradually  into  the  wider  cavity 
of  the  body.  (In  the  human  subject  the  connection  between  the  canal 
of  the  cervix  and  the  cavity  of  the  body  is  abrupt,  constituting  the 
OS  uteri  internum.) 

The  Cavity  of  the  Body  is  triangular  in  form,  with  the  base  in  front. 
At  its  posterior  angle  it  passes  into  the  canal  of  the  cervix,  and  at  each 
antero-lateral  angle  it  is  joined  by  the  cavity  of  a  horn. 

The  Cavities  of  the  Horns  are  conical  and  curved.  Each  is  widest  at 
its  base,  where  it  joins  the  cavity  of  the  body ;  and  it  tapers  to  its 
anterior  extremity,  in  the  centre  of  which  it  presents  a  small  tubercle 
perforated  by  the  uterine  orifice  of  the  Fallopian  tube. 

The  wall  of  the  uterus  comprises  serous,  musculai',  and  mucous 
layei's  : — 

1.  The  Serous  Coat  is  peritoneum,  continuous  with  the  layers  of  the 
broad  ligaments.     It  completely  envelops  the  organ. 

2.  The  Muscufar  Coat  is  composed  of  non-striped  fibres  arranged  as 


360  THE  ANATOMY  OF  THE  HORSE. 

an  external  longitndinal,  and  an  inner  circular  set.  To  compensate  for 
the  expansion  of  the  uterine  wall  during  pregnancy,  and  to  provide  a 
force  to  expel  the  fatus  at  parturition,  there  is,  during  pregnancy,  both 
an  increase  in  the  size,  and  an  addition  to  the  number,  of  these 
muscular  fibres. 

3.  The  Mucous  Coat  forms  a  complete  lining  to  the  uterus.  It  is 
smooth,  of  a  pale  pink  colour,  and  thrown  into  longitudinal  wrinkles. 
The  epithelium  is  simple,  columnar,  and  ciliated,  except  in  the  posterior 
part  of  the  cervix,  where  it  is  stratified  and  squamous,  as  in  the  vagina. 
In  the  cornua  and  body  the  mucous  membrane  is  set  with  numerous 
utricular  glands.  The  mouths  of  these  gland&,  open  on  the  surface  of 
the  membrane,  while  their  blind  ends  lie  against  the  muscular  coat. 
They  lie  obliquely  in  the  membrane,  and  are  branched  at  their  deep 
ends.     They  are  lined  by  a  single  layer  of  columnar  ciliated  cells. 

The  mucous  membrane  of  the  cervix  contains  numeroiis  mucous 
follicles,  and  the  peculiar  oimla  Nahothi,  which  appear  to  be  mucous 
glands  distended  into  a  vesicular  form  by  their  own  clear  secretion.  In 
pregnancy  these  cervical  glands  secrete  the  mucous  plug  that  closes  the 
OS  uteri. 

Directiom. — The  student  must  now  return  to  the  dissection  of  parts 
remaining  in  the  pelvis,  beginning  with  the  lumbo-sacral  plexus  (page 
349). 

THE    TAIL   (fig.   48). 

Directions. — Saw  through  the  ilium  that  is  still  intact,  making  the 
section  across  the  bone  at  the  great  sciatic  foramen.  By  cutting  the 
sacro-sciatic  ligament  on  the  same  side,  the  sacro-coccygeal  part  of  the 
spine,  with  the  sacro-iliac  joints,  will  be  isolated.  Dissect  away  the 
inferior  ilio-sacral  ligament,  and  remove  the  skin  from  the  tail. 

The  skin  of  the  tail  differs  from  that  of  the  body  in  general  in  the 
greater  length  of  its  hairs.  On  its  under  surface,  however,  extending 
backwards  from  its  root,  there  is  a  triangular  area  without  hairs. 
Along  the  under  surface  of  the  tail,  and  especially  in  front,  the  skin  is 
thin;  but  on  its  upper  aspect  and  sides  it  is  thick,  and  intimately 
adherent  to  the  subjacent  fascia. 

The  muscles  of  the  tail  are  enveloped  by  a  strong  coccycfeal  fascia 
which  is  continuous  in  front  with  the  inferior  ilio-sacral  ligament.  The 
isolation  of  the  muscles  can  be  readily  effected  near  the  root  of  the  tail, 
but  towards  its  tip  they  tend  to  blend  with  each  other.  In  each  half 
of  the  tail  there  are  three  muscles,  viz.,  one  above,  one  below,  and  one 
at  the  side,  Thei-e  are  also  three  arteries — one  on  the  middle  line 
below,  and  one  between  the  inferior  and  lateral  muscles  on  each  side. 
On  each  side  there  are  two  sets  of  nerves,  one  of  which  accompanies  the 
lateral  artery,  while  the   other   is   on  the  upper  aspect  of  the  bones, 


DISSECTION    OF    THE    PELVIS.  361 

between  the  lateral  and  superior  muscles.  [Besides  the  three  muscles 
now  to  be  described,  there  is  the  compressor  coccygis  already  dissected 
(page  343).] 

The  Erector  Coccygis  (sacro-coccygeus  superior).  This  muscle  arises 
from  the  sides  and  summits  of  the  sacral  spines,  and  it  is  inserted  by 
e\iccessive  short  tendons  to  the  upper  aspect  of  the  coccygeal  vertebrae. 

Action.— Acting  with  its  fellow,  to  elevate  the  tail  directly ;  acting 
alone,  to  elevate  the  tail  and  incline  it  laterally. 

The  CuRVATOR  Coccygis  (sacro-coccygeus  lateralis).  This  muscle 
seems  to  continue  backwards  the  semispinalis  of  the  loins.  It  arises 
from  the  last  two  lumbar  spines  and  from  the  spines  of  the  sacrum, 
and  it  is  inserted  into  the  lateral  aspect  of  the  coccygeal  bones. 

Action. — To  bend  the  tail  to  the  side  of  the  acting  muscle. 


Fig.  48. 

Muscles  of  the  Tail,  deep  Muscles  oe  the  Hip,  and  Pelvic  Licajients  {Chauccaii). 

1.  Erector  coccygis ;  2.  Curvator  coccygis ;  3.  Depressor  coccygis ;  4.  Compressor  coccygis 
5.  Deep  gluteus ;  tj.  Rectus  parvus  ;  7.  Common  tendon  of  obturator  internus  and  pyriformis 
8.  Gemelli ;  9.  Accessory  fasciculus  of  the  same  ;  10.  Quadratus  femoris ;  11.  Sacro-sciatic  ligament 
12.  Great  sacro-sciatic  foramen  ;  13.  Superior  ilio-s;icral  ligament ;  14.  Inferior  ilio-sacral  ligament. 

The  Depressor  Coccygis.  Anteriorly  this  muscle  consists  of  an 
outer  and  an  inner  portion,  Avhich  Leyh  describes  as  separate  muscles. 
It  arises  from  the  lower  face  of  the  sacrum,  beginning  about  the  3rd 
foramen.  The  slips  of  the  inner  portion  are  inserted  into  the  first  six 
coccygeal  A^ertebrse,  while  the  outer  portion  extends  to  the  extremity  of 
the  tail,  and  is  provided  with  strong  tendons  of  insertion. 

Action. — It  inclines  the  tail  laterally  or  depresses  it,  according  as  it 
acts  alone  or  with  the  opposite  muscle. 


362  THE    ANATOMY    OK    THE    HOHliE. 

Between  the  preceding  two  muscles  a  number  of  semi-independent 
fleshy  fiisciculi  connect  adjacent  coccygeal  bones.  Leyh  describes  these 
separately  as  the  inteiiran-wersales  caudce. 

At  the  root  of  the  tail,  between  the  right  and  left  depressors,  the 
retractor  muscles  of  the  penis  take  origin  from  the  1st  and  2nd  or 
2nd  and  3rd  coccygeal  bones ;  and  behind  these  the  so-called  suspensory 
ligaments  of  the  rectum  are  inserted  (Plate  46). 

The  Middle  Coccygeal  Artery  (Plate  48).  This  is  the  largest 
artery  of  the  tail.*  It  is  an  unpaired  vessel,  and  in  the  great  majority 
of  cases  it  is  a  collateral  branch  detached  from  the  inner  side  of  the 
lateral  sacral  artery  towards  the  middle  of  the  sacrum.  Sometimes 
it  is  detached  in  the  same  way  ft-om  the  left  lateral  sacral  artery. 
Passing  backwards  and  inwards  on  the  lower  surftvce  of  the  sacnim,  it 
places  itself  on  the  middle  line,  and  extends  in  that  position  throughout 
the  tail,  lying  under  the  coccygeal  vertebrae,  and  between  the  right 
and  left  depressor  muscles.  In  its  backward  course  it  gradually  reduces 
itself  by  giving  off  lateral  branches. 

The  Lateral  Coccygeal  Artery  (Plate  48).  Each  artery  (right  or 
left)  is  one  of  the  terminal  branches  of  the  lateral  sacral  artery  (the 
ischiatic  artery  being  the  other  branch).  Having  its  origin  towards  the 
middle  of  the  sacrum,  it  passes  backwards  in  the  tail,  crossing  the  sides 
of  the  coccygeal  bones,  between  the  depressor  and  curvator  muscles,  the 
fonner  muscle  separating  it  from  the  middle  artery.  It  becomes  smaller 
by  the  detachment  of  numerous  collateral  twigs,  the  largest  of  which 
pass  upwards.  Leyh  designates  this  vessel  the  infero-lateral  coccygeal 
artery,  describing  as  the  supero-lateral  coccygeal  artery  what  is, 
apparently,  an  unusually  large  branch  of  the  first. 

Veins.  The  foregoing  arteries  are  accompanied  by  veins  of  the  same 
names. 

Coccygeal  Nerves.  There  are  five  or  six  pairs  of  coccygeal  nerves, 
and  they  are  numbered  according  to  the  bones  behind  which  they  turn 
outwards,  the  fix-st  issuing  behind  the  first  coccygeal  vertebra,  and  so  on 
with  the  others.  The  first  of  them  has  a  loop  of  communication  with 
the  last  sacral.  As  they  tuini  outwai'ds,  they  divide  into  an  upper  and 
a  lower  liranch  coi-responding  to  the  superior  and  inferior  primary 
branches  of  the  spinal  nerves  in  other  regions.  The  branches  of  each 
of  these  sets  are  directed  backwards,  detaching  slender  filaments,  and 
then  applying  themselves  to  the  next  nerve  of  the  same  set.  In  this 
way  there  are  fonned  in  each  half  of  the  tail  two  composite  nerves,  one  of 
which  accompanies  the  lateral  artery,  while  the  other  runs  on  the  upper 
aspect  of  the  tail  between  the  erector  and  curvator  coccygis.  These 
cords  are  expended  in  branches  to  the  muscles  and  skin  of  the  tail. 

*  Leyh  de.scribes  and  figures  this  artery  as  being  smaller  than  the  lateral  coccygeal, 
but  that,  certainly,  is  not  usually  the  case. 


DISSECTION    OF    THE    PELVIS.  363 

Sacral  Nerves.  On  the  upper  aspect  of  the  sacrum  the  superior 
primary  branches  of  the  sacral  nerves  will  be  found  at  their  points  of 
emergence  from  the  spinal  canal,  the  first  four  issuing  by  the  superior 
sacral  foramina,  and  the  last  by  the  foramen  between  the  sacrum  and 
the  first  bone  of  the  coccyx.  These  nerves  are  much  smaller  than  the 
corresponding  inferior  primary  branches ;  and  after  giving  twigs  to  the 
muscles  on  the  side  of  the  spine,  they  pass  upwai'ds  to  the  skin  of  the 
croup.  Slender  branches  of  the  lateral  sacral  artery  issue  from  the 
spinal  canal  in  company  with  them. 

JOINTS    AND    LIGAMENTS    OP    THE    SACRUM    AND    COCCYX. 

The  sacral  portion  of  the  spine  in  the  adult  animal  does  not  present 
any  joints  between  its  constituent  pieces,  which  are  fused  by  anchylosis. 
The  lumbar  supraspinous  ligament  is  prolonged  on  the  summits  of  the 
sacral  spines.  This  region,  howevei-,  furnishes  the  important  joint 
between  the  vertebral  column  and  the  skeleton  of  the  hind  limb — the 
sacro-iliac  articulation. 

The  Sacro-iliac  Articulation.  The  bony  surfaces  that  concur  to 
form  this  are  the  auricular  facet  on  the  lateral  aspect  of  the  sacrum,  and 
the  corresponding  fecet  on  the  pelvic  surface  of  the  ilium.  The  move- 
vients  permitted  in  the  joint  are  scarcely  appreciable,  as  the  student 
may  prove  by  grasjiing  the  sacrum  and  the  part  of  the  ilium  left  in 
connection  with  it.  Since  this  joint  is  the  bond  of  connection  between 
the  skeleton  of  the  trunk  and  that  of  the  hind  limb,  in  which,  in  loco- 
motion, the  main  pi'opulsive  eftbrts  are  originated,  it  is  necessary  that 
but  slight  movement  should  be  permitted,  as  otherwise  these  effoi-ts 
would  not  be  transmitted  with  precision  to  the  trunk.  The  stability  of 
the  joint  is  effected  mainly  by  one  ligament — the  sacro-iliac,  and  to  a 
less  degree  by  the  superior  and  inferior  ilio-sacral  ligaments  and  the 
sacro-sciatic  ligament  already  described  (page  339). 

The  Sacro-iliac  Ligament. — This  ligament  is  composed  of  strong  fibres 
passing  between  the  sacrum  and  ilium,  in  close  relation  to  the  joint. 
It  consists  of  an  upper  and  a  lower  half,  corresponding  I'espectively  to 
the  anterior  and  posterior  sacro-iliac  ligaments  of  human  anatomy.  The 
former  is  much  the  stronger  of  the  two;  and  the  necessity  for  its  strength 
is  apparent  when  one  reflects  that  whatever  weight  is  placed  on  the 
back  and  loins  of  the  horse,  tends  to  drive  the  sacrum  downwards  from 
its  connection  with  the  iliac  bones,  and  that  this  tendency  is  rather 
favoured  than  otherwise  by  the  form  of  the  articular  surfaces,  which 
offer  an  aiTangement  comparable  to  an  inverted  arch. 

The  bones  should  be  disarticulated  to  show  the  articular  surfaces. 
The  joint  is  provided  with  a  rudimentary  synovial  membrane. 

Sacro-coccygeal  and  Intbr-coccygeal  Articulations.  Ordinarily 
these   are   movable  joints,  the   articular  surfaces   being   the   opposed 


36-i  THE  ANATOMY  OF  THE  HORSE. 

extremities  of  the  rudimentary  vertebral  bodies.  These  are  couuected 
by  small  intervertebral  discs,  which  are  shaped  like  a  biconcave  lens, 
since  the  bodies  of  the  vertebrae  are  here  convex  on  ])oth  extremities. 

The  bones  are  also  invested  by  a  fibrous  sheath,  which  may  be 
supposed  to  represent  the  superior  and  inferior  common  ligaments  of 
the  back  and  loins. 

Movements. — The  biconvex  form  of  the  vertebral  centra,  and  the 
suppression  of  the  different  processes  in  this  region  give  a  great  range 
and  freedom  of  movement  to  the  tail,  which,  provided  with  its  appendage 
of  hairs,  is  admirably  fitted  to  protect  the  hind  quarters  of  the  animal 
from  the  attacks  of  insects.  It  is  interesting  to  notice  the  absence  of 
the  panniculus  carnosus  over  the  area  within  which  the  tail  is  serviceable 
for  this  purpose.  In  animals  above  middle  age  it  is  not  uncommon  to 
find  the  sacro-coccygeal,  and  even  the  first  intercoccygeal  joint, 
anchvlosed. 


INDEX. 


Abdomen,  285. 
Abdominal  tunic,  288. 
Accessory  duct  of  pancreas,  ol3,  330. 
Acervulus  cerebri,  251. 
Annular  cartilage,  164. 
Annulus  ovalis,  126. 
Appendages  of  eye,  172. 
Aqueduct  of  Sylvius,  252. 
Aquednctus  vestibuli,  269. 
Aqueous  humour,  261. 
Ai-achnoid,  cranial,  235. 
,,  spinal,  137. 

Arciform  fibres  of  medulla,  239. 
Arteries — 

Anastomotic,  170,  191. 
Angular,  181. 
Anterior  abdominal,  292. 
,,        aorta,  112. 
,,         auricular,  162,  167. 
,,        circumflex,  12. 
,,         deep  temporal,  213. 
,,        dorsal  of  penis,  281. 
,,        gastric,  316. 
,,         laminal,  42. 
„        mesenteric,  304,  322. 
radial,  26. 
tibial,  77. 
Asternal,  327. 
Axillary,  4,  11,  113,  119. 

Basilar,  236. 
Brachial,  11. 
Bronchial,  105. 
Broncho-oesophageal,  118. 
Buccal,  183,  213. 

Cjecal,  305. 

Centralis  retinae,  211,  264. 
Cephalic,  119. 
Cerebellar,  237. 
Cerebral,  237. 

Cerebro-spinal,  140,  170,  236. 
Ciliary,  211. 
Circle  of  Willis,  238. 
Cireumflex  of  coronary  cushion,  41. 
Uiac,  292,  322. 
„  of  toe,  42. 

Coeliac  axis,  315,  322. 
CoUc,  305. 


ARTEmEH— continued. 
Common  aorta,  112. 

„        carotid,  148,  191. 
Coronary,  123. 

„  circle,  29,  41. 

Deep  humeral,  12. 

,,     femoral,  .59,  61. 
Digital  of  fore  limb,  28. 
hind  limb,  79. 
Dorsal,  113,  154. 
Dorso-cervical,  118. 
Dorso-spinal,  96. 
Duodenal,  316. 

External  carotid,  167,  192. 
iliac,  322. 
pudic,  290. 
,,         thoracic,  4,  113. 

Femoral,  58,  61. 
Femoro-popliteal,  61. 

Castric,  316. 
Gluteal,  6G,  351. 
Great  meningeal,  188. 

Helicinre,  284. 
Hepatic,  315,  332. 
Hypogastric,  343. 

Ileo-caecal,  305. 
Iliaco-femoral,  69,  351. 
Ilio-lumbar,  351. 
Inferior  cervical,  4,  113,  152. 
,,       communicating,  42. 

dental,  185, 188. 
„       labial,  181. 
Infraorbital,  213. 
Innominata,  118. 
Intercostal,  98,  112. 
Internal  carotid,  191 ,  237. 

iliac,  323,  3.50. 

maxillary,  167,  187,  212. 

pudic,  66,  275,  342,  350. 

thoracic,  113,  121. 
Interosseous  of  fore-arm,  22,  26. 
,,  metacarpal,  29. 


366 


THE  ANATOMY  OF  THE  HORSE. 


AKTKRIES— fOHtl  H  U(d. 
Ischiatic,  06,  350. 
Lachrymal,  175,  211. 
Large  metacarpal,  28. 
,,      metatarsal,  78. 
Lateral  coccygeal,  350,  3G2. 

„      sacral,  a^O. 
Left  gastro-omental,  317. 
Lingual,  192,  200. 
Lumbar,  99,  321. 

Mammary,  290. 
Mastoid,  170,  191. 
Maxillo-muscular,  167,  182. 
Meningeal,  188,  191,  238. 
Mental,  185. 
Middle  coccygeal,  351,  362. 

,,        sacral,  322. 

,,        spinal,  140. 

Nasal  of  ophthalmic,  220,  238. 

,,        submaxillary,  181. 
Nutrient  of  femur,  .59. 

,,  humerus,  12,  21. 

,,  ilium,  351. 

,,  metacarpal  bone,  30. 

,,  metatarsal  bone,  79. 

,,  radius,  26. 

„  scapida,  17. 

tibia,  75. 

Obturator,  62,  351. 

Occipital,  169,  191. 

Occipito-muscular,  170. 

CEsophageal,  118. 

Of  corpus  cavernosum,  282. 

Of  plantar  cushion,  29,  41. 

Of  quadriceps,  59. 

Of  spermatic  cord,  322. 

Ophthalmic,  211,  212,  238. 

Orbital,  176,  211. 

Ovarian,  322. 

Pancreatic,  316. 
Palato-labial,  180,  201,  213. 
Perforating  metatarsal,  78. 
Perpendicular,  29. 
Pharyngeal,  192,  204,  205. 
Phrenic,  321. 
Plantar  of  digital,  41. 
,,       interosseous,  79. 
of  tibial,  74,  78. 
Pleuro-oesophagea,  316. 
Popliteal,  74. 
Posterior  abdominal,  292. 

aorta,  112,  118,  321. 

,,        auricular,  163.  167. 

,,        circumflex,  16. 

,,        communicating,  237. 

,,         deep  temporal,  188. 

,,         dorsal  of  penis,  281. 

,,        gastric,  316. 

mesenteric,  306,  322,  344. 
radial,  22. 

„         tibial,  74. 
Prehumeral,  12. 
Preplantar,  41. 
Prepubic,  293,  323. 
Prevertebral,  191. 


Arteri  ES— (OH  t  in  ued. 
Profunda,  59,  61. 
Pterygoid,  186,  188. 
Pulmonary,  105,  111,  127. 
Pyloric,  316. 

Renal,  319,  322,  334. 
Retrograde  of  occipital,  170,  191. 
Right  gastro-omental,  316. 

Saphenic,  57. 
Small  metacarpal,  29. 
,,      metatarsal,  78. 
Spermatic,  279,  322. 
Spheno-palatine,  188,  213,  220. 
Spheno-spinous,  188. 
Siilenic,  316,  332. 
Staphyline,  204,  213. 
Subcarpal  arch,  29. 
Subcostal,  113. 

Subcutaneous  abdominal,  281,  286. 
Submaxillary,  170,  181, 192. 
Submental,  171,  193,  198. 
Subscapular,  11,  16. 
Superior  cerWcal,  113,  154. 

,,       dental,  213. 

,,       labial,  181. 
Superficial  temporal,  167. 
Supracarpal  arch,  28. 
Susrascapular,  11. 
Supraorbital,  175,  211. 

Thyroid,  148. 
Thyro-laryngeal,  148. 
Transverse  facial,  167,  182. 
Tympanic,  188,  269. 

Ulnar,  21. 
Umbilical,  343. 
Uterine,  323. 

Vaginal,  343. 
Vertebral,  113,  157. 
Vesico-prostatic,  342. 

Articulations  (see  joints). 
Arytenoid  cartilages,  22.5. 
Auricle,  left,  128. 

,,        right,  125. 
Auriculo-ventricular  groove,  106. 

,,  ,,  opening,  left,  129. 

right,  126. 
Axilla,  4. 

Barbs,  196. 
Bars  of  hoof,  36. 
Basilar  membrane,  271. 
Bicuspid  valve,  129. 
Bile  duct,  315,  330,  332. 
Bladder,  345,  347. 
Bowman's  capsule,  334. 
glands,  219. 
,,  membrane,  260. 

Brachial  plexus,  5,  12. 
Brain,  233. 

Brunner's  glands,  308. 
Buccal  gland,  inferior,  184. 
,,        ,,        superior,  183. 
Bulb,  238. 


367 


Caecum,  290. 

Calamus  scriptorius,  242. 
Canalis  reuniens,  270. 
Canal  of  Corti,  272. 
Petit,  266. 
Carpal  sheath,  33. 
Cartilages  of  ear,  164, 
,,  foot,  40. 

,,  larynx,  224. 

,,  nostril,  176. 

,,  trachea,  149. 

,,       tarsal,  175. 
Caruncula  lachrymalis,  173. 
CaruncuUe  myrtiformes,  3.58. 
Cavity  of  abdomen,  294. 
,,        nose,  216. 
„         pelvis,  340. 
,,         iiharynx,  206. 
„         thorax,  100. 
,,        Winslow,  300. 
Central  canal  of  spinal  cord,  142. 
Cerebellum,  240. 
Cerebrum,  242. 
Chord*  tendinese,  127,  129. 
Choroid  coat,  263. 

„        plexus  of  4th  ventricle,  242. 
,,  ,,         lateral  ventricle,  249. 

Ciliary  processes,  263. 
Circle  of  Willis,  238. 
Circumvallate  papillae,  196. 
Clitoris,  357. 
Cochlea,  270. 
Colic  mesentery,  302. 
Collecting  tubules,  334. 
Colliculus  seminalis,  348. 
Colon,  large,  296. 
„      small,  297. 
Columnje  carnese,  126,  129. 
Columns  of  cord,  142. 
Commissures  of  cord,  142. 

,,  lateral  ventricle,  252. 

Common  ejaculatory  ducts,  347. 
Conchal  cartilage,  164. 
Coni  vasculosi,  281. 
Conjunctiva,  174. 
Conus  arteriosus,  127. 
,,       medullaris,  139. 
Convoluted  tubules,  334. 
Convolutions  of  cerebrum,  245. 
Cornea,  259. 
Cornicula  laryngis,  226. 
Corona  glandis,  286. 
Coronary  cushion,  38. 
Corpora  cavernosa,  282. 
,,        geniculata,  252. 

nigra,  262. 
,,        quadrigemina,  252. 
Corpus  albicans,  243. 
„       Arantii,  127. 
„       callosum,  248, 
,,       dentatum,  241, 
,,       Highmori,  279, 
„       luteum,  354. 
,,      spongiosum,  282. 
Cowjjer's  glands,  346. 
Cricoid  cartilage,  225. 
Crico-thyroid  membrane,  226. 
Crista  acoustica,  270. 
Criira  of  cerebrum,  242. 


Criura  of  diaphragm,  327. 

,,        penis,  282. 
Crusta,  242. 
Cuticular  ridge,  329. 
Cutigeral  groove,  36. 

Dartos,  278. 

Decussation  of  jiyramids,  239. 

Descemet's  membrane,  200. 

Diaphragm,  327. 

Duct  of  Wiisung,  313,  330. 

Ducts  of  Rivinius,  197. 

Ductus  ad  nasum,  174,  177,  218. 

,,      cochlearis,  272. 

„       vestibuli,  270. 
Duodenum,  298,  312. 
Dura  mater,  cranial,  233. 
,,         ,,       spinal,  137. 

Ear,  external,  159. 

,,     internal,  269, 

„     middle,  267, 
Encephalon,  233. 
Endocardium,  125. 
Epicardium,  106. 
Epididymis,  279, 
Epiglottis,  226. 
Epigastric  region,  294. 
Epiploon,  300,  .312. 
Eustachian  tubes,  187,  206. 

valve,  126. 
External  abdominal  ring,  289, 
Eyeball,  257. 
Eyelids,  172, 

Fallopian  tubes,  354. 
Falx  cerebri,  234. 
Fascia— 

Coccygeal,  360. 

Cremasteric,  278. 

Gluteal,  64. 

Iliac,  325. 
Infundibuliform,  278. 


Of  fore-arm,  21. 
Of  thigh,  .57. 

Perineal,  275. 

Scapular,  17. 
Spermatic,  278. 
Fenestra  ovalis,  267. 

,,        rotunda,  267. 
Filiform  papillae,  196. 
Fimbrise  of  Fallopian  tubes,  3.54. 
Fissures  of  spinal  cord,  141. 

cerebrum,  243,  245,  246. 
liver,  314. 
Folia  of  cerebellum,  241. 
Foot,  35. 

Foramen  dextrum,  328. 
,,         of  Monro,  252. 
„         of  Winslow,  301. 
,,         ovale,  126. 
,,         sinistrum,  .328. 


368 


THE  ANATOMY  OF  THE  HORSE. 


Foramina  Thebesii.  li".. 
Fornix,  2'^. 
Fossa  ovalis,  126. 
Fovea  heinielliptica,  2(iit. 
,,      hemispherica,  209. 
Frwnum  lingua;,  106. 
Frog,  37. 
Fungiform  papillse,  196. 

Galactopherous  sinuses,  286. 
Ganglia— 
Andersch's,  255. 

Ciliary,  212. 

Gasserian,  254. 
Geniculate,  255. 

Inferior  cervical,  110,  117. 


Middle 
Superior 


109,  117 
195. 


Of  roots  of  spinal  nerves,  140. 

,,  root  of  vagus,  2.'S5. 

„  trunk  of     „       194. 
Otic,  189. 

Petrous,  2.55. 

Semilunar,  318. 

Sphenopalatine,  214. 
Gastric  glands,  329. 
Gastro-colic  omentum,  300,  312. 
Gastro-hepatic      ,,        312. 
Gastro-splenic      ,,        301,  312. 
Gland  of  Harder,  173. 
Glands  of  Bowman,  219. 
Glans  penis,  283. 
Glisson's  capsule,  331. 
Globus  major,  279. 
,,       minor,  279. 
Glomerulus,  334. 
Graafian  follicles,  352. 
Great  mesenterj',  302. 

,,      omentum,  300,  312. 

,,      sesamoid  sheath,  34. 

,,      transverse  fissure  of  cerebrum,  2o 
Guttural  pouches,  187. 
Gyrus  fornicatus,  247. 

Hard  palate,  200. 
Harderian  gland,  173. 
Heart,  106,  123. 
Henle's  tubes,  334. 
Hiatus  aorticus,  328. 
Hippocampus,  250. 
Hoof,  35. 

Horny  lamin.'e,  36. 
Hyaloid  membrane,  266. 
Hymen,  358. 

Hypochondriac  regions,  294. 
Hypogastric  regions,  294. 

Deo-cwcal  valve,  311. 
neum,  298. 
Iliac  region,  294. 
Incus,  268. 
Inguinal  canal,  290. 


Inferior  pyramid,  239. 

Intestines,  295. 

Internal  abdominal  ring,  293. 

Interarticular  cartilages  of  stifle-joint.  85. 

jaw,  216. 
Interpeduncular  space,  242. 
Intervertebral  substance,  136,  159. 
Iris,  261. 
Isthmus  of  fauces,  202. 

Jacobson's  organ,  219. 
Jejunum,  298. 
Joints— 

Atlanto-a.xial,  21.5. 

Basi-cornual,  207. 

Carpus,  46. 
Chondro -costal,  1.34. 
Chondro-sternal,  134. 
Costo-central,  132. 
Costo-transverse,  133. 
Crico-thyroid,  226. 
C'rico-aryteuoid,  226. 

Elbow,  44. 

Femoro-])atellar,  82. 
Femoro-tibial,  83. 
Fetlock,  50. 

Hip,  338. 
Hock,  86. 

Intercoccygeal,  363. 
Intercornual,  207. 
Intermetacarpal,  50. 
Intervertebral,  134. 

Knee,  46. 

Occipito-atlantal,  214. 

Pastern,  .52. 

Ratlio-ulnar,  45. 

Sacro-coccygeal,  363. 
Sacro-iliac,  363. 
Shoulder,  43. 
Stifle,  81. 

Tarsus,  Sii. 

Temporo-hyoideal,  207. 
Temporo-maxillary,  21.5. 
Thyro-epiglottic,  227. 
Tibio-fibular,  85. 

Kidneys,  318,  333. 

Labia  vtdvw,  3.56. 
Labial  glands,  180. 
Labyrinth,  269. 
Lachrymal  canals,  174. 

duct,  174,  177,  218 

gland,  173,  208. 
,,         puncta,  174. 

sac,  174. 
Lacteals,  306. 


369 


Lamina  cinerea,  243. 
,,       cribrosa,  259. 
Large  intestine,  295,  .309. 
Larynx,  224. 

Lateral  cartilages  of  foot,  40. 
Lens,  265. 

Lieberklihn's  glands,  308,  310. 
Ligaments — 

Anterior  calcaneo-astragaloid,  89. 
,,         common  of  knee,  48. 
,,        cuboido-cunean,  90. 
„         cuboido-scaphoid,  89. 
,,         of  elbow,  45. 

of  fetlock,  51. 
,,         tibio-tarsal,  88. 
Antero-lateral  of  coffin-joint,  53. 
Arciform,  45. 
Arcuate,  328. 
Astragalo-metatarsal,  88. 
Atlanto-axial,  215. 

Broad  of  lung,  102. 
,,         uterus,  303. 

Calcaneo-metatarsal,  89. 
-astragaloid,  89. 
Capsular,  crico-arytenoid,  227. 

crico-thyroid,  227. 

chondro-sternal,  134. 

hip,  339. 

femoro-patellar,  82. 

intertransverse  lumbar,  135. 

intervertebral,  135,  159. 

temporo-maxillary,  215. 
Carpo-metacarpal,  49. 
Chondro-xiphoid,  134. 
Coronary  of  liver,  315. 
Costo-transverse,  133. 
Costo-vertebral,  132. 
Cotyloid,  339. 
Crucial  of  stifle,  84. 
Cruciform,  214. 

Denticulated,  138. 

External  temporo-maxillary,  215. 

Falciform  of  liver,  315. 

Gastro-phrenic,  312. 

Hepatic,  314. 

Inferior  atlanto-axial,  215, 
„      of  bladder,  342. 
„       common,  135. 
,,      ilio-sacral,  MO. 
„       sesamoidean,  51, 
Interannular,  215. 
Interarticular,  133. 
Intercarpal,  48. 
Intercunean,  90. 

Interosseous  calcaneo-astragaloid,  89. 
,,  costo-transverse,  133. 

,,  cuboido-cunean,  90. 

,,  cuboido-scaphoid,  90. 

,,  of  coffin-joint,  53. 

,,  radio-ulnar,  45. 

„  tarso-metatarsal,  90. 


Ligaments — continued. 

Interosseous  tibio-fibular,  86. 
Intersesamoid,  51. 
Interspinous,  135,  159. 
Intervertebral  substance,  136,  159. 

Lateral  of  bladder,  342. 
,,  elbow,  44,  45. 

,,     femoro-patellar,  82. 
,,     femoro-tibial,  83. 
of  fetlock,  51. 
,,  of  knee,  46,  48. 

,,  of  liver,  314. 

,,  of  pastern,  52. 

,,     sesamoidean,  51. 
,,     tibio-tarsal,  86. 

Nuchse,  158. 

Occipito-atlantal,  214. 

Patellar,  82. 

Posterior  calcaneo-astragaloid,  89. 

,,         common  of  knee,  48. 

,,         femoro-tibial,  84. 

,,        temporo-maxillary,  215. 

,,         tibio-tarsal,  88. 
Postero-lateral  of  coffin-joint,  53. 
Pubio-femoral,  339. 

Radio-carpal,  48. 
Round,  339. 

Sacro-iliac,  363. 
Sacro-sciatic,  339. 
Scaphoido-cunean,  90. 
Splenic,  313. 
Stellate,  132. 
Straight  patellar,  82. 
Styloid,  214. 
Subcarpal,  35,  48. 
Subflava,  135,  159. 
Subtarsal,  81,  89. 
Superior  atlanto-axial,  215. 

,,       common,  1.35,  159. 

,,       ilio-sacral,  340. 

,,       sesamoidean,  50. 
Supraspinous,  1.35. 
Suspensory  of  liver,  314. 
penis,  282. 
,,  prepuce,  281. 

,,  rectum,  276,  344. 

Tarso-metatarsal,  89. 
Thyro-arytenoid,  227. 
Tibio-tarsal,  86. 

Uterine  broad,  303. 
Linea  alba,  289. 
Lingual  fibrous  cord,  197. 

„       glands,  197. 
Liver,  313,  330. 
Locus  cperuleus,  240. 

„     niger,  243. 

,,     perforatus  anticus,  243. 

,,  ,,         posticus,  243. 

Lumbar  region,  294. 
Lungs,  103, 122. 
Lunula,  127. 

9    R 


370 


THE  ANATOMY  OF  THE  HORSE. 


Lymphatics— 
Brachial,  11. 
Bronchial,  115. 

Cardiac,  115. 

Iliac,  321. 
Inguinal,  deep,  .57. 

,,        superficial,  287. 
Intestinal,  306. 

Lumbar,  321. 

Olsophageal,  115. 
Of  liver,  315. 
Of  spleen,  315. 
Of  stomach,  315. 

Pharyngeal,  191. 
Popliteal,  61. 
Precrural,  57. 
Prepectoral,  115,  149. 
Prescapular,  152. 

Sacral,  321. 
Submaxillary,  170. 
Suprasternal,  122. 

Thoracic,  115. 
Tracheal,  115. 
Malleus,  268. 
Mammary  glands,  285. 
Malpighian  bodies  of  kidney,  334. 
spleen,  333. 
Meatuses  of  nose,  216. 
Meatus  urinarius,  358. 
Mediastinum  of  thorax,  102. 

testis,  279. 
Medulla  oblongata,  238. 
Membrana  nictitans,  173. 
„  pupillaris,  262. 

,,  reticularis,  272. 

,,  tectoria,  272. 

„  tympani,  267. 

Membranes  of  brain,  233. 

,,  spinal  cord,  137. 

Mesentery,  302. 
Meso-rectum,  342. 
Mitral  valve,  129. 
Moderator  bands,  126. 
Modiolus,  270. 
Movements  of  joints,  43. 
Muscles — 
Adductor  magnus,  GO. 
,,         parvus,  60. 
Accelerator  urina;,  277. 
Anconeus,  18. 
Anterior  deep  pectoral,  3. 

,,         superficial  pectoral,  2. 
Arytenoideus,  230. 
Aryteno-pharyngeus,  205. 
Azygos  uvulte,  204. 

Biceps,  15. 
Biceps  femoris,  64. 
Brachialis  anticus,  18. 
Buccinator,  179. 

Caput  magnum,  15. 


Muscles — continued. 
Caput  medium,  18. 
,,      parvum,  15. 
Cerato-hyoid,  205. 
Cervico-auriculares,  160. 
Ciliary,  262. 
Complexus,  154. 
Compressor  coccygis,  343. 
Constrictors  of  viUva,  356. 
Coraco-humeralis,  15. 
Corrugator  supercilii,  175. 
Cremaster,  278,  294. 
Crico-arytenoideus  lateralis,  229. 
Crico-arytenoideus  posticus,  229. 
Crico-pharyngeus,  205. 
Crico-thyroideus,  228. 
Crureus,  62. 
Curvator  coccygis,  361. 

Deep  flexor  of  digit  (fore  limb), 

24.  34,  42. 
Deep  flexor  of  digit  (hind  limb), 

73,  81. 
Deep  gluteus,  67. 
Deltoid,  17. 
Depressor  labii  inferior  is,  179. 

,,         labii  sui)erioris,  180. 

,,         coccygis,  361. 
Diaphragm,  327. 
Digastricus,  106,  171,  190. 
Dilatator  uaris  inferior,  179. 

,,         naris  lateralis,  178. 

,,         naris  superior,  178. 

,,        naris  transversalis,  178. 
pupillae,  262. 

Erector  clitoridis,  357. 
,,       coccygis,  361. 
„       penis,  277. 
Extensor  metacarpi  magnus,  24. 
,,  ,,         obliquus,  25. 

pedis  (fore  limb),  25,  32. 
„     (hind  limb),  75,  80. 
,,        suflfraginis,  25,  32. 
External  intercostal,  t)8. 
,,       pterygoid,  185. 


accessorius,  73. 
metacarpi  externus,  23. 

,,         internus,  23. 

,,         medius,  23. 
metatarsi,  76. 
pedis  perforans  (fore  limb), 
,34. 

pedis  perforans  (hind  limb), 
,81. 

pedis  perforatus  (fore  limb), 
,34. 

pedis  iierforatus  (hind  limb), 
,  8L 


Flexor 


24, 
Flexor 

73. 
Flexor 

23, 
Flexor 

72, 


Gastrocnemius,  71. 
Gemelli,  68. 
Genio-glossus,  200. 
Genio-hyoideus,  200. 
Gracilis,  58. 
Great  hyo-glossus,  199. 

Hyo-epiglottideus,  228. 


371 


^IV&CLEfi— continued. 

Hyoideus  transversiis,  199. 
Hyo-pliaryngeus,  205. 

Iliacus,  63,  326. 
Inferior  oblique  of  eye,  210. 
Infraspinatus,  17. 
Internal  intercostal,  98. 
,,        pterygoid,  185. 
Interossei,  3'2,  80. 
Intertransversales  of  loins,  326. 
,,  ,,  neck,  155. 

Iscliio-urethral,  346. 

Lateralis  sterni,  97. 

Latissimus  dorsi,  9,  14,  94. 

Levator  anguli  scapulae,  8. 

Levatores  costarum,  97. 

Levator  labii  superioris  al?eq\ie  nasi, 

177. 
Levator  labii  superioris  proprius, 

178. 
Levator  menti,  180. 
palati,  204. 
,,        palpebrse  superioris,  175. 
Longissimus  dorsi,  96. 
Longus  colli,  156. 
Lumbricales,  31,  80. 

Masseter,  180. 
Mastoido-auricularis,  162. 
Mastoido-humeralis,  10,  152,  166. 
Middle  glutevis,  65. 

,,       hyo-glossus,  199. 
Mylo-hyoideus,  171,  198. 

Obliquus  abdominis  externus,  289. 

,,  ,,  internus,  291. 

,,       capitis  inferior,  169. 

,,  ,,       superior,  169. 

Obturator  externus,  61. 

,,  internus,  67. 

Occipito-styloid,  166. 
Orbicularis  oris,  179. 

,,  palpebrarum,  175. 

Palato-glossus,  199. 
Palato-pharyngeus,  203. 
Panniculus,  8,  94,  144,  170,  177, 

287. 
Parieto-auricularis  externus,  160. 
,,  ,,  internus,  162. 

Parotido-auricularis,  160. 
Pectineus,  60. 
Peroneus,  75,  81. 
Popliteus,  72. 
Posterior  deep  pectoral,  3. 
Posterior  superficial  pectoral,  2. 
Psoas  magnus,  63,  325. 

,,      parvus,  326. 
Pterygo-pharyngeus,  205, 
Pyriformis,  67. 

Quadratus  femoris,  61. 

,,         lumborum,  .326. 

Recti  oculi,  209. 
Rectus  abdominis,  292. 

,,     capitis  anticus  major,  155. 


MuscLKS — continued. 

Rectus  capitis  anticus  minor,  208. 
„  „      lateralis,  208. 

,,  ,,      posticus  major,  169. 

,,  ,,      i^osticiis  minor,  169. 

,,      femoris,  69. 
,,      parvus,  69. 
Retractor  ani,  276. 
,,         costse,  96. 
„         oculi,  209. 
,,         penis,  276. 
Rhomboideus,  10. 

Sartorius,  57. 
Scalenus,  156. 

Scapulo-immeralis  gracilis,  16. 
Scapulo-ulnaris,  14. 
Scuto-auricularis  externus,  162. 
,,  ,,         internus,  162. 

Semimembranosus,  60. 
Semispinalis  of  back  and  loins,  97. 

colli,  155. 
Semitendinosiis,  65. 
Serratus  anticus,  95. 

,,         magnus,  7,  95. 

,,         posticus,  95. 
Short  extensor  of  foot,  80. 
Small  liyo-glossus,  199. 

,,      stylo-pliaryngeus,  205. 
Soleus,  71. 
Sphincter  ani,  276. 

pupillee,  262. 
Splenius,  153. 
Stapedius,  268. 
Sterno-maxillaris,  145,  166. 
Sterno-thyro-hyoideus,  146. 
Stylo-glossus,  198. 
Stylo-hyoideus,  166,  190. 
Stylo-maxillaris,  166. 
Stylo-pharyngeus,  205. 
Subscapularis,  15. 
Subscapulo-hyoideus,  146. 
Superficial  flexor  of  digit  (fore  limb), 

23,  34. 
Superficial  flexor  of  digit  (hind  limb), 

72,  81. 
Superficial  gluteus,  64. 
Superior  oblique  of  eye,  210. 
Supraspinatus,  17. 

Temporalis,  185. 
Tensor  palati,  203. 

,,       tympani,  268. 

,,       vaginse  femoris,  64. 
Teres  major,  14. 

,,     minor,  17. 
Thyro-arytenoid,  229. 
Thyro-hyoid,  228. 
Thyro-pharyngeus,  205. 
Trachelo-mastoid,  153. 
Transversalis  abdominis,  293. 

,,  costarum,  96. 

Transversus  perinfei,  276. 
Trapezius,  9. 
Triangularis  sterni,  120. 
Triceps  extensor  cubiti,  15. 

Vastus  externus,  69. 
internus,  62. 


372 


THE  ANATOMY  OP  THE  HORSE. 


Muscles — continued. 
Wilson's  muscle,  34G. 

Zygomatico-auricularis,  161. 
Zygomaticus,  179. 
Musculi  papillares,  127,  129. 
„       pectinati,  125,  128. 

Nares,  inferior,  176,  218. 
,,      superior,  206,  218. 
Nates,  252. 
Navicular  sheath,  42. 

Nerves— 
Abducent,  212,  254. 
Anterior  auricular,  163. 

„      crural,  59,  325,  349. 
„      tibial,  77. 
Auditory,  255,  272. 
Auricular  of  1st  cervical,  164. 
2nd      „        164. 
„         7th  cranial,  163,  168. 

10th     „        255. 
,,         lachrymal,  164. 

Brachial  plexus,  5,  12,  109. 
Buccal,  184,  189. 

Cardiac,  110,  117,  124. 

Cervical  of  7th  cranial,  144,  163,  168. 

spinal,  152,  154,  169,  195. 
Chorda  tympani,  189,  269. 
Ciliary,  212,  263. 
Circumflex,  13. 
Coccygeal,  362. 
Cranial,  253. 
Cremasteric,  324. 
Cutaneous  of  chest-wall  and  back,  94. 

,,  fore-arm,  20. 

„  hip  and  thigh,  63. 

, ,  intermaxillary  space,  170. 

leg,  70. 

,,  metatarsus,  78. 

neck,  143,  151. 

,,  iJectoral  region,  1. 

,,  perinaeum,  275. 

,,  prepuce,  281. 

„  thigh,  57. 

Digital,  31. 
Dorsal  of  penis,  281. 
,,      spinal,  96,  99 

Eighth  cranial,  255,  272. 

Eleventh  cranial,  140,   146,   151,  194, 

255. 
External  laryngeal,  194. 

popliteal,  62,  07,  74,  76. 

,,       saphenous,  62,  67,  74. 

Facial,  168,  176,  182,  254. 
Fifth  cranial,  254. 
First  cranial,  219,  253. 
Fourth  cranial,  212,  254. 

Glosso-pharyngeal,  193,  200,  206,  255. 
Gluteal,  66,  349. 
Great  sciatic,  62,  67,  349. 
Gustatory,  186,  198. 


Nerves — continued. 

Haemorrhoidal,  275,  343. 
Hypoglossal,  193,  198,  256. 

Iliaco-muscular,  349. 
Inferior  dental,  186,  189. 

„       laryngeal,  left,  109,  149,  230. 

„  „        right,  116,  149,  230. 

,,       maxillary,  188,  254. 
Infra-orbital,  183. 
Inguinal,  281,  291,  324. 
Intercostal,  99,  292. 
Internal  popliteal,  73. 

,,        saphenous,  57,  325. 

Jacobson's,  193,  255. 

Lachrymal,  211. 

Large  superficial  petrosal,  214. 

Lingual,  186,  189. 

Lumbar,  96,  99,  292,  324. 

Lumbo-sacral  jilexus,  324,  349. 

Median,  14,  22. 

Mental,  186. 

IMiddle  auricular,  163. 

Muscido-cutaneous  of  median,  14. 

„  „  popliteal,  76. 

Musculo-spiral,  13,  26. 
Motor  oculi,  176,  212,  254. 
Mylo-hyoid,  172,  186,  189. 

Nasal,  220. 

Ninth  cranial,  193,  200,  206,  255. 

Obturator,  62,  349. 

Oesophageal,  317. 

Olfactory,  219,  253. 

Ophthalmic,  211,  254. 

Optic,  253. 

Orbital  of  superior  maxillary,  176,  211. 

Palatine,  202,  213. 
Palpebro-nasal,  176,  211. 
Pathetic,  212,  254. 
Perforating,  288. 
Peroneal  cutaneous,  67. 
Pharyngeal,  of  9th,  193,  206. 
of  10th,  194. 
,,  of  sympathetic,  195. 

Phrenic,  6,  108,  116. 
Plantar,  of  fore  limb,  30. 
of  hind  limb,  80. 
Pneumogastric,  108,  116,  194,  255. 
Posterior  auricular,  163. 

tibial,  74. 
Portio  dura,  168,  176,  182,  254. 
,,       intermedia,  254. 
„      moUis,  255,  272. 
Pudic,  275,  343. 
Pulmonary,  109. 

Kecurrent  laryngeal,  109, 116, 149,  230. 

Sacral,  349. 

Sacral,  ;363. 

Second  cranial,  253. 

Seventh  cranial,  168,  176,  182,  254. 

Sixth  cranial,  212,  254. 


373 


Nerves — continued. 

Small  superficial  petrosal,  189. 
Spheno-palatine,  214,  220. 
Spinal,  138. 

accessoiy,  140,  146,  151,  194, 

255. 
Splanchnic,  111,  318. 
Staphyline,  204,  214. 
Subcutaneous  thoracic,  7,  288. 
Subzygomatic,  168,  183,  188. 
Superior  dental,  213. 

„        laryngeal,  194,  230. 
maxillary,  213,  254. 
Supra-orbital,  176,  211. 
Suprascapiilar,  13. 
Sympathetic  cord,  cervical,  149,  195. 

„  ,,  dorsal,  111,  118. 

„  ,,         lumbar,  325, 

,,  ,,         sacral,  350. 

Tenth  cranial,  108,  116,  194,  255. 
Third  cranial,  176,  212,  254. 
Trifacial  or  trigeminal,  254. 
Trochlear,  212,  254. 
Twelfth,  193,  198,  256 

Ulnar,  13,  21. 

Vagus,  108,  116,  194,  255. 

Vertebral,  157. 

Vidian,  214. 
Nipple,  285. 
Nodulus  Arantii,  127. 
Nostrils,  176. 

CEsophagus,  115,  150. 
Olfactory  bulbs,  244. 
cells,  219. 
Olivary  body,  239. 
Omentum,  gastro-hepatic,  312. 

,,  gastro-splenic,  301,  312. 

great,  300,  312. 
Optic  thalami,  251. 

,,  tracts,  243,  252. 
Orbicular  process,  268, 
Organ  of  Corti,  272. 

,,       Jacobson,  219, 

,,       Rosenmliller,  354, 
Osseous  spiral  lamina,  271, 
Os  uteri,  359. 
Otic  ganglion,  189. 
Otoliths,  270. 
Ovaries,  351. 
Oviducts,  354. 
Oviila  Nabothi,  360. 
Ovum,  352. 

Pacchionian  bodies,  2:34. 
Palate,  hard,  200. 
,,      soft,  202. 
Palpebral  tendon,  175. 
Papillae  of  tongue,  196, 
Papilla  optica,  264. 
Pancreas,  313,  333. 
Parotid  gland,  164. 
Parovarium  354. 
Pelvis,  338. 

Peduncles  of  cerebellum,  241. 
,,  pineal  gland,  251. 


Pedunculated  hydatid  of  morgagni, 

Penis,  281. 

Pericardium,  105. 

Perilymph,  270. 

Perinaeum,  274. 

Periople,  36. 

Perioplic  ring,  38. 

Peritoneum,  298. 

pockets  of,  303, 
Pes  anserinus,  182. 
Peyer's  patches,  309. 
Pharynx,  204. 
Pia  mater,  cranial,  236. 
,,  spinal,  138. 

Pillars  of  soft  palate,  202. 
Pineal  gland,  251. 
Pituitary  gland,  243. 

,,         membrane,  219. 
Plantar  cvishion,  38. 
Pleurae,  101. 
Plexuses  of  Nerves — 

Anterior  mesenteric,  307. 

Aortic,  325. 

Carotid,  238. 
Cavernous,  238. 
Cceliac,  318. 

Pelvic,  344. 

Posterior  mesenteric,  307. 

Eenal,  320. 

Solar,  318. 

Spermatic,  325. 

Suprarenal,  318. 
Pomum  Adami,  225. 
Pons  Tarini,  243. 

„     Varolii,  240. 
Portal  fissure,  314. 
Poupart's  ligament,  289. 
Prepuce,  280. 
Prostate,  346. 
Puncta  lachrymalia,  174. 
Pupil,  261. 
Pylorus,  311. 
Pyramidal  body,  40. 
Pyramids  of  Ferrein,  334. 
medulla,  239. 

Quadrilateral  space,  244. 

Eeceptaculum  chyli,  324. 

Kectum,  297,  344. 

Reissner's  membrane,  271. 

Eenal  crest,  334. 

Eete  testis,  280. 

Retina,  264. 

Right  lymphatic  duct,  119. 

Rivinius,  ducts  of,  197. 

Root  of  hing,  105. 

Roots  of  cranial  nerves,  253. 

,,       spinal  nerves,  140. 
Rostrum,  248. 

Saccule,  270. 

Scala  intermedia,  272. 

,,     tympani,  271. 

„     vestibuli,  272. 


374 


THE  ANATOMY  OF  THE  HORSE. 


Schneiderean  membrane,  243. 
Sclerotic,  259. 
Scrotum,  277. 
Scutiform  cartilage,  1()4. 
Semicircular  canals,  270. 
Semilunar  cartilages  of  stifle,  8.5. 
Seminal  tubules,  280. 
Sensitive  frog,  38. 
,,         lamin?e,  38. 
sole,  40. 
Septum  lucidum,  249. 
,,        nasi,  217. 
„        i)ectiniforme,  283. 
„        scroti,  278. 
Sheath,  280. 

Sinuses  of  diu-a  mater,  234. 
head,  219. 
,,  larynx,  231. 

valsalva,  128,  129. 
Sinus,  subsplienoidal,  207. 

,,       venosus,  107. 
Small  intestines,  29.5,  308. 
,,      sesamoid  sheath,  42. 
Soft  palate,  202. 
Sole  of  hoof,  36. 
Solitary  glands,  308. 
Spermatic  cord,  278,  294. 
Spermatoblasts,  280. 
Spinal  cord,  130. 

,,      nerves,  139. 
Spleen,  312,  332. 
Splenium,  248. 
Stapes,  268. 
Staphyline  glands,  203. 
Stenson's  duct,  165,  171,  182,  184. 

„        canal,  219. 
Stomach,  311,  328. 
Striae  longitudinales,  249. 
Subarachnoid  space,  138,  235. 
Subdural  space,  137,  235. 
Sublingual  gland,  197. 
ridge,  196. 
Submaxillary  gland,  190. 
Subsphenoidal  sinus,  207. 
Suburethral  notch,  283. 
Suprarenal  capsules,  320. 

TaU,  361. 

Tienia  hippocampi,  250. 
Tapetum  lucidum,  263. 
Teat,  285. 
Tegmentum,  242. 
Tentorium  cerebelli,  234. 
Testes,  252. 
Testicle,  279. 
Thoracic  cavity,  100. 

duct,  114. 
Thymus  gland,  115. 
Thyroid  body,  147. 

,,        cartilage,  225. 
Tongue,  195. 
Tonsil,  202. 

Torcular  Herophili,  235. 
Trabeculse  carnete,  126. 

of  spleen,  332. 
Trachea,  115,  149. 
Trapezium,  239. 
Tricuspid  valve,  127. 
Tuber  cinereum,  243. 


Tubercle  of  Lower,  125. 

Tubuli  recti,  280. 

Tunica  albuginea,  279,  283. 

,,       Kuyscliiana,  263. 

„      vaginalis,  278,  279. 

,,       vasculosa,  280. 
Tympanum,  267. 

Udder,  285. 
Umbilical  region,  294. 
Ureters,  320,  345. 
Urethra,  female,  358. 

male,  282,  345,  347. 
Urethral  sinus,  283. 
,,       tube,  283. 
Urinary  bla«lder,  345,  347. 
Uriniferous  tubules,  334. 
Uterus,  355,  359. 

,,      masculinus,  346. 
Utricle,  270. 
Utricular  glands,  360. 
Uvea,  262. 

Vagina,  356,  ^58. 

V.^VLVES — 

Auriculo-ventricular,  left,  129. 
right,  127. 

Bicuspid,  129. 

Eustachian,  126. 

Mitral,  129. 

Of  meatus  urinarius,  .358. 
Of  Thebesius,  125. 
Of  Vieussens,  242. 

Semilunar  aortic,  129. 

„  pulmonary,  127. 

Tricuspid,  127. 

Vasa  deferentia,  279,  346. 
,,     efferentia,  280. 
,,     vorticosa,  263. 
Veins — 
Alveolar,  183,  213. 
Angular,  181. 
Anterior  auricular,  163. 
,,       gastric,  317. 
„       mesenteric,  306,  317. 
,,        subcutaneous  of  fore-arm,  20. 

tibial,  77. 
,,       vena  cava,  114,  119. 
Asternal,  327. 
Axillary,  5. 
Azygos,  great,  119. 
„       small,  114. 

Brachial,  12. 
Bronchial,  105. 
Buccal,  183. 

Cephalic,  2,  145. 
Circumflex  iliac,  292. 
Common  iliac,  323. 
Cordis  minimae,  125. 
Coronary  of  heart,  124. 


375 


Veins — continued. 
Coronary  plexus,  42. 

Digital,  of  fore  limb,  30. 
„  hind  limb,  7'J. 

Dorsal,  114,  119,  155. 

External^iliac,  323. 
,,        pudic,  291. 

Femoral,  59,  61. 

Gastric,  317. 
Gastro-omental,  317. 
Great  vena  azygos,  119. 

Hsemorrhoidal,  344. 
Hepatic,  323,  332. 

Inferior  dental,  186. 
Intercostal,  99,  119. 
Internal  iliac,  323,  351. 

maxillary,  168,  186,  188. 
pudic,  276,  343. 
,,        subcutaneous  of  fore-arm,  20. 
,,        saphena,  57,  70. 

thoracic,  114,  119,  122. 

Jugular,  144,  167. 

Laminal  plexus,  42. 
Lateral  coccygeal,  362. 
Lingual,  186,  198. 
Lumbar,  99,  119,  323. 

I\Iaxillo-muscular,  168,  182. 
Median,  20. 
Metacarpal,  30. 
Metatarsal,  79. 
Middle  coccygeal,  362. 

Obturator,  62. 
Occipital,  170,  193. 
Of  Galen,  251. 
Ophthalmic,  211,  213. 

Palatine,  202,  213. 
Phrenic,  323. 
Popliteal,  75. 
Portal,  317,  331. 


Veins — continued. 

Posterior  abdominal,  292. 

,,         auricular,  163,  168. 

„         gastric,  317. 

„         mesenteric,  306,  317,  344. 

,,         radial,  22. 

,,         tibial,  75. 

,,         vena  cava,  119,  323. 
Pterygoid,  186. 
Pulmonary,  105. 

Renal,  320. 

Small  vena  azygos,  114. 
Solar  plexus  of  foot,  42. 
Spermatic,  279,  323. 
Spinal,  141. 

Spheno-palatine,  213,  220. 
Splenic,  317. 
Spur,  287. 
Staphyline,  204. 
Subcutaneous  abdominal,  287. 
,,  thoracic,  287. 

Vasa  vorticosa,  263. 
Velum  interpositum,  251. 

,,       pendulum  palati,  202. 
Ventricle  of  brain,  fourth,  241. 

lateral,  249. 

third,  252. 
Ventricle  of  heart,  left,  128. 

right,  126. 
Ventricles  of  larynx,  231. 
Ventricular  grooves,  106. 
Vermiform  lobe,  240. 
Verumontanum,  348. 
Vesiculte  seminales,  346,  348. 
Vesicula  j)rostatica,  346. 
Vestibular  bulb,  357. 
Vestibule  of  ear,  269. 
Villi  of  intestine,  308. 
Vitreous  humour,  266. 
Vocal  cord,  227. 
Vulva,  356,  357. 

Wall  of  hoof,  36. 
Wharton's  duct,  190,  197. 
Wirsung's  duct,  313. 

Zonula  of  Zinn,  265. 


